Dissertations / Theses on the topic 'Variational problem'
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Brading, Katherine. "Symmetries, conservation laws and Noether's variational problem." Thesis, University of Oxford, 2001. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.288912.
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Arceci, Francesca. "Variational algorithms for image Super Resolution." Master's thesis, Alma Mater Studiorum - Università di Bologna, 2019. http://amslaurea.unibo.it/19509/.
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La Super Resolution è una tecnica che permette di aumentare la risoluzione di un’immagine oltre i limiti imposti dai sensori. Nel processo di acquisizione e formazione dell’immagine, vi sono infatti fenomeni di noise e blurring che la corrompono: da qui l’esigenza di ricostruire l’input reale. Una volta modellizzato questo processo, vi sono svariate tecniche SR che approcciano in modi differenti al problema: in questo lavoro ci basiamo su teniche reconstruction-based che prevedono la minimizzazione di due funzionali, uno che misura la coerenza tra dato e soluzione, l’altro è un termine di regolarizzazione.
Lo studio di questa tesi si basa su un’immagine con gradiente sparso, più precisamente un QR code: partendo dalla descrizione del modello matematico, il nostro lavoro sarà quello di trovare un funzionale di regolarizzazione che esprima la proprietà del gradiente sparso e, basandoci sull’approccio dell’ Alternating Direction Method of Multipliers, implementare un nuovo algoritmo che risolva il problema di minimo ad esso associato. Mostreremo i risultati ottenuti confrontandoli con algoritmi preesistenti per provarne la buona performance.
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Haben, Stephen A. "Conditioning and preconditioning of the minimisation problem in variational data assimilation." Thesis, University of Reading, 2011. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.541945.
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Chi, Xuguang. "A non-variational approach to the quantum three-body coulomb problem /." View abstract or full-text, 2004. http://library.ust.hk/cgi/db/thesis.pl?PHYS%202004%20CHI.
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Fiscella, A. "VARIATIONAL PROBLEMS INVOLVING NON-LOCAL ELLIPTIC OPERATORS." Doctoral thesis, Università degli Studi di Milano, 2014. http://hdl.handle.net/2434/245334.
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My thesis deals with nonlinear elliptic problems involving a non-local integrodifferential operator of fractional type. Our main results concern the existence of weak solutions for these problems and they are obtained using variational and topological methods.
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Salavessa, Isabel. "Graphs with parallel mean curvature and a variational problem in conformal geometry." Thesis, University of Warwick, 1987. http://wrap.warwick.ac.uk/99902/.
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This thesis essentially deals with two basic problems, one in Rieinannian, the other in Conformal Geometry, described in Part I resp. Part III. Part II can be considered as an interlude serving as a sort of bridge between Riemannian and Comformal Geometry.
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Agnihotri, Mayank P. "One particle properties in the 2D Coulomb problem Luttinger-Ward variational approach /." kostenfrei, 2007. http://www.digibib.tu-bs.de/?docid=00020957.
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Köhler, Karoline Sophie. "On efficient a posteriori error analysis for variational inequalities." Doctoral thesis, Humboldt-Universität zu Berlin, Mathematisch-Naturwissenschaftliche Fakultät, 2016. http://dx.doi.org/10.18452/17635.
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Effiziente und zuverlässige a posteriori Fehlerabschätzungen sind eine Hauptzutat für die effiziente numerische Berechnung von Lösungen zu Variationsungleichungen durch die Finite-Elemente-Methode. Die vorliegende Arbeit untersucht zuverlässige und effiziente Fehlerabschätzungen für beliebige Finite-Elemente-Methoden und drei Variationsungleichungen, nämlich dem Hindernisproblem, dem Signorini Problem und dem Bingham Problem in zwei Raumdimensionen. Die Fehlerabschätzungen hängen vom zum Problem gehörenden Lagrange Multiplikator ab, der eine Verbindung zwischen der Variationsungleichung und dem zugehörigen linearen Problem darstellt. Effizienz und Zuverlässigkeit werden bezüglich eines totalen Fehlers gezeigt. Die Fehleranschätzungen fordern minimale Regularität. Die Approximation der exakten Lösung erfüllt die Dirichlet Randbedingungen und die Approximation des Lagrange Multiplikators ist nicht-positiv im Falle des Hindernis- und Signoriniproblems, und hat Betrag kleiner gleich 1 für das Bingham Problem. Dieses allgemeine Vorgehen ermöglicht das Einbinden nicht-exakter diskreter Lösungen, welche im Kontext dieser Ungleichungen auftreten. Aus dem Blickwinkel der Anwendungen ist Effizienz und Zuverlässigkeit im Bezug auf den Fehler der primalen Variablen in der Energienorm von großem Interesse. Solche Abschätzungen hängen von der Wahl eines effizienten diskreten Lagrange Multiplikators ab. Im Falle des Hindernis- und Signorini Problems werden postive Beispiele für drei Finite-Elemente Methoden, der konformen Courant Methode, der nicht-konformen Crouzeix-Raviart Methode und der gemischten Raviart-Thomas Methode niedrigster Ordnung hergeleitet. Partielle Resultate liegen im Fall des Bingham Problems vor. Numerischer Experimente heben die theoretischen Ergebnisse hervor und zeigen Effizienz und Zuverlässigkeit. Die numerischen Tests legen nahe, dass der aus den Abschätzungen resultierende adaptive Algorithmus mit optimaler Konvergenzrate konvergiert.Efficient and reliable a posteriori error estimates are a key ingredient for the efficient numerical computation of solutions for variational inequalities by the finite element method. This thesis studies such reliable and efficient error estimates for arbitrary finite element methods and three representative variational inequalities, namely the obstacle problem, the Signorini problem, and the Bingham problem in two space dimensions. The error estimates rely on a problem connected Lagrange multiplier, which presents a connection between the variational inequality and the corresponding linear problem. Reliability and efficiency are shown with respect to some total error. Reliability and efficiency are shown under minimal regularity assumptions. The approximation to the exact solution satisfies the Dirichlet boundary conditions, and an approximation of the Lagrange multiplier is non-positive in the case of the obstacle and Signorini problem and has an absolute value smaller than 1 for the Bingham flow problem. These general assumptions allow for reliable and efficient a posteriori error analysis even in the presence of inexact solve, which naturally occurs in the context of variational inequalities. From the point of view of the applications, reliability and efficiency with respect to the error of the primal variable in the energy norm is of great interest. Such estimates depend on the efficient design of a discrete Lagrange multiplier. Affirmative examples of discrete Lagrange multipliers are presented for the obstacle and Signorini problem and three different first-order finite element methods, namely the conforming Courant, the non-conforming Crouzeix-Raviart, and the mixed Raviart-Thomas FEM. Partial results exist for the Bingham flow problem. Numerical experiments highlight the theoretical results, and show efficiency and reliability. The numerical tests suggest that the resulting adaptive algorithms converge with optimal convergence rates.
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El-Said, Adam. "Conditioning of the weak-constraint variational data assimilation problem for numerical weather prediction." Thesis, University of Reading, 2015. http://centaur.reading.ac.uk/45568/.
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4-Dimensional Variational Data Assimilation (4DVAR) assimilates observations through the minimisation of a least-squares objective function, which is constrained by the model flow. We refer to 4DVAR as strong-constraint 4DVAR (sc4DVAR) in this thesis as it assumes the model is perfect. Relaxing this assumption gives rise to weak-constraint 4DVAR (wc4DVAR), leading to a different minimisation problem with more degrees of freedom. We consider two wc4DVAR formulations in this thesis, the model error formulation and state estimation formulation. The 4DVAR objective function is traditionally solved using gradient-based iterative methods. The principle method used in Numerical Weather Prediction today is the Gauss-Newton approach. This method introduces a linearised `inner-loop' objective function, which upon convergence, updates the solution of the non-linear `outer-loop' objective function. This requires many evaluations of the objective function and its gradient, which emphasises the importance of the Hessian. The eigenvalues and eigenvectors of the Hessian provide insight into the degree of convexity of the objective function, while also indicating the difficulty one may encounter while iterative solving 4DVAR. The condition number of the Hessian is an appropriate measure for the sensitivity of the problem to input data. The condition number can also indicate the rate of convergence and solution accuracy of the minimisation algorithm. This thesis investigates the sensitivity of the solution process minimising both wc4DVAR objective functions to the internal assimilation parameters composing the problem. We gain insight into these sensitivities by bounding the condition number of the Hessians of both objective functions. We also precondition the model error objective function and show improved convergence. We show that both formulations' sensitivities are related to error variance balance, assimilation window length and correlation length-scales using the bounds. We further demonstrate this through numerical experiments on the condition number and data assimilation experiments using linear and non-linear chaotic toy models.
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Vedovato, Mattia. "Some variational and geometric problems on metric measure spaces." Doctoral thesis, Università degli studi di Trento, 2022. https://hdl.handle.net/11572/337379.
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In this Thesis, we analyze three variational and geometric problems, that extend classical Euclidean issues of the calculus of variations to more general classes of spaces. The results we outline are based on the articles [Ved21; MV21] and on a forthcoming joint work with Nicolussi Golo and Serra Cassano. In the first place, in Chapter 1 we provide a general introduction to metric measure spaces and some of their properties. In Chapter 2 we extend the classical Talenti’s comparison theorem for elliptic equations to the setting of RCD(K,N) spaces: in addition the the generalization of Talenti’s inequality, we will prove that the result is rigid, in the sense that equality forces the space to have a symmetric structure, and stable. Chapter 3 is devoted to the study of the Bernstein problem for intrinsic graphs in the first Heisenberg group H^1: we will show that under mild assumptions on the regularity any stationary and stable solution to the minimal surface equation needs to be intrinsically affine. Finally, in Chapter 4 we study the dimension and structure of the singular set for p-harmonic maps taking values in a Riemannian manifold.
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Lilli, Markus. "The effect of a singular perturbation to a 1-d non convex variational problem." Berlin Logos-Verl, 2004. http://deposit.ddb.de/cgi-bin/dokserv?id=2752886&prov=M&dok_var=1&dok_ext=htm.
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Lilli, Markus. "The effect of a singular perturbation to a 1-d non convex variational problem /." Berlin : Logos-Verl, 2005. http://deposit.ddb.de/cgi-bin/dokserv?id=2752886&prov=M&dok_var=1&dok_ext=htm.
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Ghaderi, Hazhar. "Mountain Pass Theorems with Ekeland’s Variational Principle and an Application tothe Semilinear Dirichlet Problem." Thesis, Uppsala universitet, Analys och tillämpad matematik, 2011. http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-168198.
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SOAVE, NICOLA. "Variational and geometric methods for nonlinear differential equations." Doctoral thesis, Università degli Studi di Milano-Bicocca, 2014. http://hdl.handle.net/10281/49889.
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This thesis is devoted to the study of several problems arising in the field of nonlinear analysis. The work is divided in two parts: the first one concerns existence of oscillating solutions, in a suitable sense, for some nonlinear ODEs and PDEs, while the second one regards the study of qualitative properties, such as monotonicity and symmetry, for solutions to some elliptic problems in unbounded domains. Although the topics faced in this work can appear far away one from the other, the techniques employed in different chapters share several common features. In the firts part, the variational structure of the considered problems plays an essential role, and in particular we obtain existence of oscillating solutions by means of non-standard versions of the Nehari's method and of the Seifert's broken geodesics argument. In the second part, classical tools of geometric analysis, such as the moving planes method and the application of Liouville-type theorems, are used to prove 1-dimensional symmetry of solutions in different situations.
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Money, James H. "Variational methods for image deblurring and discretized Picard's method." Lexington, Ky. : [University of Kentucky Libraries], 2006. http://lib.uky.edu/ETD/ukymath2006d00415/DISSERT.PDF.
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Thesis (Ph. D.)--University of Kentucky, 2006.Title from document title page (viewed on May 31, 2006). Document formatted into pages; contains x, 97 p. : ill. (some col.). Includes abstract and vita. Includes bibliographical references (p. 90-96).
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Gürol, Selime. "Solving regularized nonlinear least-squares problem in dual space with application to variational data assimilation." Thesis, Toulouse, INPT, 2013. http://www.theses.fr/2013INPT0040/document.
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Cette thèse étudie la méthode du gradient conjugué et la méthode de Lanczos pour la résolution de problèmes aux moindres carrés non-linéaires sous déterminés et régularisés par un terme de pénalisation quadratique. Ces problèmes résultent souvent d'une approche du maximum de vraisemblance, et impliquent un ensemble de m observations physiques et n inconnues estimées par régression non linéaire. Nous supposons ici que n est grand par rapport à m. Un tel cas se présente lorsque des champs tridimensionnels sont estimés à partir d'observations physiques, par exemple dans l'assimilation de données appliquée aux modèles du système terrestre. Un algorithme largement utilisé dans ce contexte est la méthode de Gauss- Newton (GN), connue dans la communauté d'assimilation de données sous le nom d'assimilation variationnelle des données quadridimensionnelles. Le procédé GN repose sur la résolution approchée d'une séquence de moindres carrés linéaires optimale dans laquelle la fonction coût non-linéaire des moindres carrés est approximée par une fonction quadratique dans le voisinage de l'itération non linéaire en cours. Cependant, il est bien connu que cette simple variante de l'algorithme de Gauss-Newton ne garantit pas une diminution monotone de la fonction coût et sa convergence n'est donc pas garantie. Cette difficulté est généralement surmontée en utilisant une recherche linéaire (Dennis and Schnabel, 1983) ou une méthode de région de confiance (Conn, Gould and Toint, 2000), qui assure la convergence globale des points critiques du premier ordre sous des hypothèses faibles. Nous considérons la seconde de ces approches dans cette thèse. En outre, compte tenu de la grande échelle de ce problème, nous proposons ici d'utiliser un algorithme de région de confiance particulier s'appuyant sur la méthode du gradient conjugué tronqué de Steihaug-Toint pour la résolution approchée du sous-problème (Conn, Gould and Toint, 2000, p. 133-139) La résolution de ce sous-problème dans un espace à n dimensions (par CG ou Lanczos) est considérée comme l'approche primale. Comme alternative, une réduction significative du coût de calcul est possible en réécrivant l'approximation quadratique dans l'espace à m dimensions associé aux observations. Ceci est important pour les applications à grande échelle telles que celles quotidiennement traitées dans les systèmes de prévisions météorologiques. Cette approche, qui effectue la minimisation de l'espace à m dimensions à l'aide CG ou de ces variantes, est considérée comme l'approche duale. La première approche proposée (Da Silva et al., 1995; Cohn et al., 1998; Courtier, 1997), connue sous le nom de Système d'analyse Statistique de l'espace Physique (PSAS) dans la communauté d'assimilation de données, commence par la minimisation de la fonction de coût duale dans l'espace de dimension m par un CG préconditionné (PCG), puis revient l'espace à n dimensions. Techniquement, l'algorithme se compose de formules de récurrence impliquant des vecteurs de taille m au lieu de vecteurs de taille n. Cependant, l'utilisation de PSAS peut être excessivement coûteuse car il a été remarqué que la fonction de coût linéaire des moindres carrés ne diminue pas monotonement au cours des itérations non-linéaires. Une autre approche duale, connue sous le nom de méthode du gradient conjugué préconditionné restreint (RPCG), a été proposée par Gratton and Tshimanga (2009). Celle-ci génère les mêmes itérations en arithmétique exacte que l'approche primale, à nouveau en utilisant la formule de récurrence impliquant des vecteurs taille m. L'intérêt principal de RPCG est qu'il en résulte une réduction significative de la mémoire utilisée et des coûts de calcul tout en conservant la propriété de convergence souhaitée, contrairement à l'algorithme PSASThis thesis investigates the conjugate-gradient method and the Lanczos method for the solution of under-determined nonlinear least-squares problems regularized by a quadratic penalty term. Such problems often result from a maximum likelihood approach, and involve a set of m physical observations and n unknowns that are estimated by nonlinear regression. We suppose here that n is large compared to m. These problems are encountered for instance when three-dimensional fields are estimated from physical observations, as is the case in data assimilation in Earth system models. A widely used algorithm in this context is the Gauss-Newton (GN) method, known in the data assimilation community under the name of incremental four dimensional variational data assimilation. The GN method relies on the approximate solution of a sequence of linear least-squares problems in which the nonlinear least-squares cost function is approximated by a quadratic function in the neighbourhood of the current nonlinear iterate. However, it is well known that this simple variant of the Gauss-Newton algorithm does not ensure a monotonic decrease of the cost function and that convergence is not guaranteed. Removing this difficulty is typically achieved by using a line-search (Dennis and Schnabel, 1983) or trust-region (Conn, Gould and Toint, 2000) strategy, which ensures global convergence to first order critical points under mild assumptions. We consider the second of these approaches in this thesis. Moreover, taking into consideration the large-scale nature of the problem, we propose here to use a particular trust-region algorithm relying on the Steihaug-Toint truncated conjugate-gradient method for the approximate solution of the subproblem (Conn, Gould and Toint, 2000, pp. 133-139). Solving this subproblem in the n-dimensional space (by CG or Lanczos) is referred to as the primal approach. Alternatively, a significant reduction in the computational cost is possible by rewriting the quadratic approximation in the m-dimensional space associated with the observations. This is important for large-scale applications such as those solved daily in weather prediction systems. This approach, which performs the minimization in the m-dimensional space using CG or variants thereof, is referred to as the dual approach. The first proposed dual approach (Courtier, 1997), known as the Physical-space Statistical Analysis System (PSAS) in the data assimilation community starts by solving the corresponding dual cost function in m-dimensional space by a standard preconditioned CG (PCG), and then recovers the step in n-dimensional space through multiplication by an n by m matrix. Technically, the algorithm consists of recurrence formulas involving m-vectors instead of n-vectors. However, the use of PSAS can be unduly costly as it was noticed that the linear least-squares cost function does not monotonically decrease along the nonlinear iterations when applying standard termination. Another dual approach has been proposed by Gratton and Tshimanga (2009) and is known as the Restricted Preconditioned Conjugate Gradient (RPCG) method. It generates the same iterates in exact arithmetic as those generated by the primal approach, again using recursion formula involving m-vectors. The main interest of RPCG is that it results in significant reduction of both memory and computational costs while maintaining the desired convergence property, in contrast with the PSAS algorithm. The relation between these two dual approaches and the question of deriving efficient preconditioners (Gratton, Sartenaer and Tshimanga, 2011), essential when large-scale problems are considered, was not addressed in Gratton and Tshimanga (2009)
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Yu, Haofeng. "A Numerical Investigation Of The Canonical Duality Method For Non-Convex Variational Problems." Diss., Virginia Tech, 2011. http://hdl.handle.net/10919/29095.
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This thesis represents a theoretical and numerical investigation of the canonical duality theory, which has been recently proposed as an alternative to the classic and direct methods for non-convex variational problems. These non-convex variational problems arise in a wide range of scientific and engineering applications, such as phase transitions, post-buckling of large deformed beam models, nonlinear field theory, and superconductivity. The numerical discretization of these non-convex variational problems leads to global minimization problems in a finite dimensional space.
The primary goal of this thesis is to apply the newly developed canonical duality theory to two non-convex variational problems: a modified version of Ericksen's bar and a problem of Landau-Ginzburg type. The canonical duality theory is investigated numerically and compared with classic methods of numerical nature. Both advantages and shortcomings of the canonical duality theory are discussed. A major component of this critical numerical investigation is a careful sensitivity study of the various approaches with respect to changes in parameters, boundary conditions and initial conditions.Ph. D.
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Scrivanti, Gabriele Luca Giovanni. "Nonsmooth Nonconvex Variational Reconstruction for Electrical Impedance Tomography." Master's thesis, Alma Mater Studiorum - Università di Bologna, 2020. http://amslaurea.unibo.it/20700/.
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Electrical Impedance Tomography is an imaging technique that aims to reconstruct the inner conductivity distribution of a medium starting from a set of measured voltages registered by a series of electrodes that are positioned on the surface of the medium. Such technique was used for the first time in geological studies in 1930 and then applied to industrial procedures. The first clinical use of EIT dates back to 1987. In 2018 EIT was validated in tissue engineering as a noninvasive and label-free imaging and monitoring tool for cell distribution (cell growth, differentiation and tissue formation) in 3D scaffolds. EIT problem can be split into a Forward and an Inverse problem. The aim of Forward EIT is to define the set of measured voltages starting from a known conductivity distribution. If the forward problem is characterized by a nonlinear mapping, called Forward Operator, from the conductivity distribution to the measured voltages, inverse EIT consists of inverting the Forward Operator. This leads to an ill-posed problem which requires regularization, either in the model or in the numerical method that is applied to define the solution. The inverse problem is modelled as a Nonlinear Least Squares problem, where one seeks to minimize the mismatch beetween the measured voltages and the ones generated by the reconstructed conductivity. Reconstruction techniques require the introduction of a regularization term which forces the reconstructed data to stick to certain prior information. In this dissertation, some state-of-the-art regularization methods are analyzed and compared via EIDORS, a specific software for EIT problems. The aim is to reconstruct the variation in conductivity within a 2D section of a 3D scaffold. Furthermore a variational formulation on a 2D mesh for a space-variant regularization is proposed, based on a combination of high order and nonconvex operators, which respectively seek to recover piecewise inhomogeneous and piecewise linear regions.
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Pont, Ribas Arnau. "Numerical simulation of aeroacoustics using the variational multiscale method : application to the problem of human phonation." Doctoral thesis, Universitat Politècnica de Catalunya, 2018. http://hdl.handle.net/10803/461955.
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The solution of the human phonation problem applying computational mechanics is covered by several research branches, such as Computational Fluid Dynamics (CFD), biomechanics or acoustics, among others. In the present thesis, the problem is approached from the Computational Aeroacoustics (CAA) point of view and the first main objective consists in developing numerical methods of general application that can take part in the solution of any scenario related to human phonation with a reasonable cost. In this sense, only the compressible Navier-Stokes equations can describe all flow and acoustic scales without any modeling, which is known as Direct Numerical Simulation (DNS), but its computational cost is usually unaffordable. Even in the case of a Large Eddy Simulation (LES), where the small scales are modeled, the cost can still be a handicap due to the complexity of the problem. This drawback gets worse in the low Mach regime due to the large disparity between flow velocity and sound speed, which leads to an ill-conditioning of the system of equations, specially for conservative schemes. At this point, it makes sense to move towards the incompressible flow approximation, bearing in mind the low velocities expected in human phonation problems.
Incompressible flows do not yield any acoustics, for which a second problem containing the propagation of the sound sources needs to be modeled and solved. These are the so called hybrid methods, which allow a better conditioning of the problem by segregating flow and acoustic scales. Lighthill's analogy has been taken as starting point for the present work, but its restriction to free-field scenarios has motivated the extension of the method to arbitrary geometries and non-uniform flows. The first development in this direction consists in a splitting of Lighthill's analogy into a quadrupolar and dipolar component, which does not change the original problem but allows assessing the contribution of solid boundaries to the generation of sound. The second step consists in the development of a stabilized Finite Element (FEM) formulation for the Acoustic Perturbation Equations (APE) which account for non-uniform flows and perform a complete filtering of the acoustic scales. The final step assumes the compressible approach but omitting the energy equation and thus considering both flow and acoustic propagation as isentropic. In this case the solver is unified and hence a method for applying compatible boundary conditions for flow and acoustics has been developed. Moreover, the whole numerical framework has been extended to dynamic phonation cases, which require using an Arbitrary Lagrangian Eulerian (ALE) reference. Also, a novel remeshing strategy with conservative interpolation between meshes is presented.
In the last chapter a challenging case in human phonation has been chosen for testing the developed computational framework: the fricative phoneme /s/. Unlike vowels, which are voiced sounds defined by a few characteristic frequencies, fricatives cannot be simulated as the propagation of a known analytic solution (glottal pulse) because the sound sources correspond to a wide range of turbulent scales. Therefore, a CFD calculation is mandatory in order to capture all relevant eddies behind the generation of sound. This problem is solved with an LES together with the Variational Multiscale (VMS) stabilization method as turbulence model, which is supplemented with several acoustic formulations when using incompressible flow. The analysis of the results focuses on the numerical representation of turbulence and the acoustic signal at the far-field, which has been compared to experimental recordings. Finally, the role of the upper incisors in the generation of the fricative sound has been evaluated. All simulations have been run with the parallel multiphysics FEM code FEMUSS, based on FORTRAN Object-Oriented-Programming land the OpenMPI parallel library.La solució del problema de la veu humana des de la mecànica computacional és objecte d'estudi per part de diverses disciplines, com per exemple la Dinàmica de Fluids Computacional (CFD), la biomecànica o l'acústica. En la present tesi s'encara el problema des de l'Aeroacústica Computacional (CAA) i el primer objectiu consisteix en desenvolupar mètodes numèrics d'aplicació general que puguin ser part de la solució, amb un cost computacional raonable, de qualsevol escenari relacionat amb la fonació humana. En aquest sentit, només les equacions de flux compressible de Navier-Stokes aconsegueixen descriure totes les escales alhora, tant les dinàmiques com les acústiques, sense recórrer a cap modelització, conegut com a Simulació Numèrica Directa (DNS), però el seu cost computacional és normalment inassumible. Fins i tot en el cas d'una Large Eddy Simulation (LES), on les escales petites són modelades, el cost pot resultar excessiu a causa de la complexitat del problema. Aquest fet encara és més accentuat en el règim de baix nombre de Mach donada la gran disparitat entre la velocitat del fluid i la del so i el conseqüent mal condicionament del sistema d'equacions, sobretot en esquemes conservatius. Per tant, tenint en compte les baixes velocitats de l'aire al tracte vocal, té sentit recórrer a l'aproximació de flux incompressible. Els fluids incompressibles no inclouen la part acústica, de manera que cal calcular un segon problema que descrigui la propagació de les fonts de so. Aquests són els anomenats mètodes híbrids, que permeten un millor condicionament del problema gràcies a la segregació de les escales acústiques de les dinàmiques. S'ha pres l'analogia de Lighthill com a punt de partida, però la seva restricció a casos en camp obert ha motivat l'extensió del mètode cap a geometries arbitràries i fluxos no uniformes. El primer desenvolupament en aquesta direcció consisteix en la divisió de l'analogia de Lighthill en una component quadrupolar i una altra de dipolar, fet que no altera el problema original però que permet analitzar la contribució de cossos sòlids en la generació de so. El segon pas consisteix en el desenvolupament d'una formulació estabilitzada en elements finits (FEM) de les Acoustic Perturbation Equations (APE), que incorporen la propagació en fluxos no uniformes i que realitzen un filtrat complet de les escales acústiques. El pas final assumeix la compressibilitat del fluid però omet l'equació d'energia, i per tant considera la dinàmica i l'acústica fenòmens isentròpics. En aquest cas el solver és unificat i per tant s'ha desenvolupat un mètode per imposar condicions de contorn compatibles entre ambdues escales del fluid. Finalment, les formulacions numèriques han estat adaptades a casos de fonació dinàmica mitjançant una referència Arbitrària Lagrangiana Euleriana (ALE). A més, es presenta una estratègia de remallat amb interpolació conservativa entre malles. En l'últim capítol es presenta un cas de fonació humana que suposa un repte per la seva complexitat i que ha servit per validar les formulacions numèriques presentades: la fricativa sorda /s/. A diferència de les vocals, que són sons sonors definits per unes poques freqüències característiques, les fricatives no poden ser simulades com la propagació d'una funció analítica coneguda (pols glotal) perquè les fonts de so corresponen a un rang ampli d'escales turbulents. Per tant és necessària una simulació CFD per tal de capturar-les. El problema se soluciona amb un model de turbulència LES amb el mètode d'estabilització Variational Multiscale. L'anàlisi se centra en la representació numèrica de la turbulència i en el senyal acústic al camp llunyà, tot comparant-lo amb dades experimentals. Finalment, s'avalua la contribució dels incisius superiors en la generació del so fricatiu sord /s/. Totes les simulacions han estat realitzades amb el codi FEM multi-físic en paral·lel FEMUSS, basat en programació orientada a objectes en FORTRAN i en OpenMPI.
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Pachas, Daniel Alexis Gutierrez. "Inequações variacionais e aplicações em problemas tipo obstáculo com resolução numérica via complementaridade." Universidade Federal de Juiz de Fora (UFJF), 2013. https://repositorio.ufjf.br/jspui/handle/ufjf/2366.
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CAPES - Coordenação de Aperfeiçoamento de Pessoal de Nível Superior
Neste trabalho realizamos um estudo teórico das Inequações Variacionais e sua aplicação no Problema do Obstáculo. Fazemos o estudo de regularidade para este problema, e observamos que quando as condições de regularidade são satisfeitas, o Problema do Obstáculo torna-se um Problema de Complementaridade. Apresentamos os resultados de equivalência entre o Problema do Obstáculo e o Problema do Dique Retangular. Descrevemos o funcionamento do Algoritmo FDA-NCP, e resolvemos numericamente o Problema do Obstáculo usando complementaridade.
In this work, we perform a theoretical study on Variational Inequalities and their application to the Obstacle Problem. We study the regularity for this problem, and observe that when the regularity conditions are satis ed the Obstacle Problem becomes a Complementarity Problem. We present the equivalence results between the Obstacle Problem and the Square Dam Problem. We describe how the algorithm FDA-NCP works and numerically to solve the Obstacle Problem employing complementarity.
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Silva, Junior Valter Aparecido 1989. "Soluções do problema de Liouville-Gelfand via grupos de Lie." [s.n.], 2015. http://repositorio.unicamp.br/jspui/handle/REPOSIP/306723.
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Orientador: Yuri Dimitrov BozhkovDissertação (mestrado) - Universidade Estadual de Campinas, Instituto de Matemática Estatística e Computação Científica
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Resumo: Nesta dissertação, obteremos as soluções exatas do Problema de Liouville-Gelfand (em uma e em duas dimensões) via grupos de Lie de simetrias
Abstract: In this dissertation, we shall obtain the exact solutions of the Liouville-Gelfand Problem (in one and in two dimensions) via Lie groups of symmetries
Mestrado
Matematica Aplicada
Mestre em Matemática Aplicada
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Batista, Edvaldo Elias de Almeida. "Generalized vector equilibrium problems and algorithms for variational inequality in hadamard manifolds." Universidade Federal de Goiás, 2016. http://repositorio.bc.ufg.br/tede/handle/tede/6562.
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Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - CAPES
In this thesis, we study variational inequalities and generalized vector equilibrium problems. In Chapter 1, several results and basic definitions of Riemannian geometry are listed; we present the concept of the monotone vector field in Hadamard manifolds and many of their properties, besides, we introduce the concept of enlargement of a monotone vector field, and we display its properties in a Riemannian context. In Chapter 2, an inexact proximal point method for variational inequalities in Hadamard manifolds is introduced, and its convergence properties are studied; see [7]. To present our method, we generalize the concept of enlargement of monotone operators, from a linear setting to the Riemannian context. As an application, an inexact proximal point method for constrained optimization problems is obtained. In Chapter 3, we present an extragradient algorithm for variational inequality associated with the point-to-set vector field in Hadamard manifolds and study its convergence properties; see [8]. In order to present our method, the concept of enlargement of maximal monotone vector fields is used and its lower-semicontinuity is established to obtain the convergence of the method in this new context. In Chapter 4, we present a sufficient condition for the existence of a solution to the generalized vector equilibrium problem on Hadamard manifolds using a version of the KnasterKuratowski-Mazurkiewicz Lemma; see [6]. In particular, the existence of solutions to optimization, vector optimization, Nash equilibria, complementarity, and variational inequality is a special case of the existence result for the generalized vector equilibrium problem.
Nesta tese, estudamos desigualdades variacionais e o problema de equilíbrio vetorial generalizado. No Capítulo 1, vários resultados e definições elementares sobre geometria Riemanniana são enunciados; apresentamos o conceito de campo vetorial monótono e muitas de suas propriedades, além de introduzir o conceito de alargamento de um campo vetorial monótono e exibir suas propriedades em um contexto Riemanniano. No Capítulo 2, um método de ponto proximal inexato para desigualdades variacionais em variedades de Hadamard é introduzido e suas propriedades de convergência são estudadas; veja [7]. Para apresentar o nosso método, generalizamos o conceito de alargamento de operadores monótonos, do contexto linear ao contexto de Riemanniano. Como aplicação, é obtido um método de ponto proximal inexato para problemas de otimização com restrições. No Capítulo 3, apresentamos um algoritmo extragradiente para desigualdades variacionais associado a um campo vetorial ponto-conjunto em variedades de Hadamard e estudamos suas propriedades de convergência; veja [8]. A fim de apresentar nosso método, o conceito de alargamento de campos vetoriais monótonos é utilizado e sua semicontinuidade inferior é estabelecida, a fim de obter a convergência do método neste novo contexto. No Capítulo 4, apresentamos uma condição suficiente para a existência de soluções para o problema de equilíbrio vetorial generalizado em variedades de Hadamard usando uma versão do Lema Knaster-Kuratowski-Mazurkiewicz; veja [6]. Em particular, a existência de soluções para problemas de otimização, otimização vetorial, equilíbrio de Nash, complementaridade e desigualdades variacionais são casos especiais do resultado de existência do problema de equilíbrio vetorial generalizado.
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Sousa, Júnior José Ribamar Alves de. "O cálculo variacional e o problema da braquistócrona /." Rio Claro : [s.n.], 2010. http://hdl.handle.net/11449/94359.
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Orientador: Suzinei Aparecida Siqueira MarconatoBanca: Renata Zotin Gomes de Oliveira
Banca: Sueli Mieko Tanaki Aki
Resumo: Neste trabalho estudamos o problema da Braquistócrona de duas formas distintas: através da teoria do Cálculo Variacional para problemas com fronteiras xas e também através das considerações feitas por Johann Bernoulli, utilizando conceitos de Óptica e Geometria. Apresentamos também uma simulação computacional dos resultados obtidos
Abstract: In this work we study the Brachistochrone Problem of two di erent ways; by theory of Variational Calculus for problems with xed boundary and by considerations of Johann Bernoulli, with concepts of Optics and Geometry. A computational simulation of the obtained results, is presented too
Mestre
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Svensson, Anton. "Non-smooth and variational analysis of optimization problems and multi-leader-follower games." Thesis, Perpignan, 2020. http://www.theses.fr/2020PERP0003.
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Cette thèse, dont le cadre général est l'optimisation, traite de problèmes d'optimisation non-lisse et de problèmes de théorie des jeux. Elle est constituée de quatre parties. Dans la première, nous présentons le contexte et l'introduction. Dans la deuxième partie, nous discutons quelques règles de calcul sous-différentiel dans des espaces généraux, et présentons notamment certaines formules plus fortes que l'état de l'art, autant dans le cas convexe que dans le cas non convexe. L'accent est mis sur les règles de calcul et conditions d'optimalité approchées et "fuzzy", pour lesquelles aucune condition de qualification n'est requise. Dans la troisième partie, nous considérons des jeux bi-niveaux à plusieurs meneurs et plusieurs suiveurs. Après quelques résultats d'existence dans le cas d'un seul meneur optimiste et dans le cas de plusieurs meneurs, nous étendons des résultats existants concernant la relation entre le problème bi-niveau original et sa reformulation obtenue grâce au remplacement des problèmes des suiveurs par la concaténation de leurs conditions d'optimalité (KKT). Finalement, dans la quatrième partie, nous abordons quelques problèmes de quasi-équilibre, qui sont une généralisation des problèmes d'équilibre de Nash et des inégalités quasi-variationnelles. Nous prouvons ainsi de nouveaux résultats d'existence qui permettent de relâcher les hypothèses standardThis thesis is within the framework of optimization and deals with nonsmooth optimization and with some problems of game theory. It is divided into four parts. In the first introductory part, we give the context and some preliminary results. In the second part we discuss about subdifferential calculus rules in general spaces providing of some improved formulas in both the convex and the non-convex cases. Here the focus is on approximate or fuzzy calculus rules and optimality conditions, for which no qualification conditions are required. In the third part, we discuss about the so-called Multi-Leader-Follower Games. We give an existence result for the case of a single optimistic leader and multiple followers, and extend some results concerning the relation between the original problem with the reformulation obtained by replacing the followers' problem by the concatenation of their KKT conditions. Finally, in the fourth part we study quasi-equilibrium problems which are a general formulation for studying Nash equilibrium problems and quasi-variational inequalities. We provide some new existence results that relax some of the standard hypotheses
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Sousa, Júnior José Ribamar Alves de [UNESP]. "O cálculo variacional e o problema da braquistócrona." Universidade Estadual Paulista (UNESP), 2010. http://hdl.handle.net/11449/94359.
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sousajunior_jra_me_rcla.pdf: 1174734 bytes, checksum: cbdf2669884098c54b72817cfc625edd (MD5)Neste trabalho estudamos o problema da Braquistócrona de duas formas distintas: através da teoria do Cálculo Variacional para problemas com fronteiras xas e também através das considerações feitas por Johann Bernoulli, utilizando conceitos de Óptica e Geometria. Apresentamos também uma simulação computacional dos resultados obtidos
In this work we study the Brachistochrone Problem of two di erent ways; by theory of Variational Calculus for problems with xed boundary and by considerations of Johann Bernoulli, with concepts of Optics and Geometry. A computational simulation of the obtained results, is presented too
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Keller, Diana [Verfasser], Wilfried [Akademischer Betreuer] Grecksch, and Björn [Akademischer Betreuer] Schmalfuß. "An optimal control problem for the stochastic nonlinear Schrödinger equation in variational formulation / Diana Keller. Betreuer: Wilfried Grecksch ; Björn Schmalfuß." Halle, Saale : Universitäts- und Landesbibliothek Sachsen-Anhalt, 2015. http://d-nb.info/1078898413/34.
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Flemming, Jens. "Quadratic Inverse Problems and Sparsity Promoting Regularization." Doctoral thesis, Universitätsbibliothek Chemnitz, 2018. http://nbn-resolving.de/urn:nbn:de:bsz:ch1-qucosa-232402.
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Ill-posed inverse problems with quadratic structure are introduced, studied and solved. As an example an inverse problem appearing in laser optics is solved numerically based on a new regularized inversion algorithm. In addition, the theory of sparsity promoting regularization is extended to situations in which sparsity cannot be expected and also to equations with non-injective operators.
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Colajanni, Gabriella. "Constrained Optimization Problems in Network Models." Doctoral thesis, Università di Catania, 2019. http://hdl.handle.net/10761/4105.
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Operations Research is the field of mathematics that deals with solving various application problems. Constrained optimization problems are one of the most important and useful fields of mathematics, particularly in Operations Research.
In this thesis, we focus our attention on some mathematical models that are decision problems and which are all based on networks and applied to different real situations.
We analyze different thematic areas such as Cloud Computing, Financial Market, Business Management and Cybersecurity and for each of them we formulate the associated linear or nonlinear constrained problems which allows us to solve the decision problems related to the specific applications.
The purpose of one of our mathematical models, in this thesis, is to represent a cloud environment. This mathematical model could allows us to identify a rational strategy for reaching a final goal, which is to maximize the Iaas provider's profit. We get a mixed-Integer nonlinear programming problem, which can be solved through the proposed computational algorithm. A second step is the linearization of the problem. The effectiveness of the model and of the algorithm is tested, by comparing the final data with the results obtained by solving the linearized problem through an existing software.
Another topic we have dealt with in depth in this thesis is the financial market. We studied some optimization models based on networks which allow us to formulate two new multi-period portfolio selection problems as Markowitz mean-variance optimization problems with intermediaries, and therefore with transaction costs, the addition of capital gains tax, but also with short selling and transfer of securities.
We proposed two constrained Integer nonlinear programming problems with which it is possible to establish if and when it is suitable to buy and to sell financial securities, not only while maximizing the profits, but also while minimizing the risk (through the use of a weight). We applied the Lagrange theory and analyzed the variational inequality studying an optimization model for business management and cybersecurity investments.
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Almeida, Samuel Oliveira de. "Soluções para problemas elípticos envolvendo o expoente crítico de Sobolev." Universidade Federal de Juiz de Fora, 2013. https://repositorio.ufjf.br/jspui/handle/ufjf/1468.
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CAPES - Coordenação de Aperfeiçoamento de Pessoal de Nível Superior
Neste trabalho estudamos a existência de soluções para problemas elípticos envolvendo o expoente crítico de Sobolev. Primeiramente, investigamos a existência de soluções para um problema superlinear do tipo Ambrosetti-Prodi com ressonância em 1, onde 1 é o primeiro autovalor de (−Δ,1 0 (Ω)). Além disso, estudamos resultados de multiplicidade para uma classe de equações elípticas críticas relacionadas com o problema de Brézis-Nirenberg, com condição de contorno de Neumann sobre a bola.
In this work we study the existence of solutions for elliptic problems involving critical Sobolev exponent. Firstly we investigate the existence of solutions for an Ambrosetti-Prodi type superlinear problem with resonance at 1 , where 1 is the first eigenvalue of (−Δ,1 0 (Ω)). Besides, we study multiplicity results for a class of critical elliptic equations related to the Brézis-Nirenberg problem with Neumann boundary condition on a ball.
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Cavalcante, Thiago Rodrigues. "Existência de uma solução não trivial para uma classe de problemas elípticos super quadrático." Universidade Federal de Goiás, 2013. http://repositorio.bc.ufg.br/tede/handle/tde/2968.
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Conselho Nacional de Pesquisa e Desenvolvimento Científico e Tecnológico - CNPq
In this dissertation we analyze questions of existence of a weak solution for a class of superlineares elliptic Dirichlet problems. Here we do not consider the Ambrosseti Rabinovitz condition , which restricts some nonlinearities. We obtain main results of this dissertation via Variational Methods, such as Mountain Pass Theorem and Linking Theorem. Furthermore, weusePalais-Smalecondition(P.S.) or Cerami condition(Ce)
Nesta dissertação analisamos questões de existência de uma solução fraca para uma classe de problemas de Dirichlet elípticos superlineares. Aqui não consideramos a condição deAmbrosetti-Rabinowitz,a qual restringealgumasfunçõesnão lineares. Obtemos os principais resultados desta dissertação via Métodos variacionais, tais como o Teorema do Passo da Montanha e um Teorema de Linking. Além disso, utilizamos a TeoriaEspectral e ascondições dePalais-Smale(P.S.) eCerami(Ce).
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Thibaut, Jérôme. "Corrélations, intrication et dynamique des systèmes quantiques à N Corps : une étude variationnelle." Thesis, Lyon, 2019. http://www.theses.fr/2019LYSEN021/document.
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Cette thèse porte sur l'étude de systèmes quantiques à N-corps à température nulle, où le comportement du système n'est alors soumis qu'aux effets quantiques. Je vais présenter ici une approche variationnelle développée avec Tommaso Roscilde, mon directeur de thèse, et Fabio Mezzacapo, mon co-encadrant de thèse, pour étudier ces systèmes.Cette approche se base sur une parametrisation de l’état quantique (dit Ansatz) à laquelle on applique une procédure d’optimisation variationnelle lui permettant de reproduire l'évolution d'un système soumis à l'équation de Schrödinger, tout en limitant le nombre de variables considérées. En considérant une évolution en temps imaginaire, il est possible d'étudier l'état fondamental d'un système. Je me suis ainsi intéressé à un modèle de chaîne XX de spins 1/2, dont les corrélations à longue portée rendent l'étude difficile, et adapté ainsi notre approche pour reproduire au mieux les corrélations et l'intrication du système. Je me suis ensuite intéressé au modèle J1-J2 dont la structure de signe non positive des coefficients de l’état quantique pose un défi important pour les approches Monte Carlo; et dans laquelle la frustration magnétique induit une transition de phase quantique (d’un état aux corrélations à longue porté vers un état non magnétique avec formation d’un cristal de lien de valence). Je me suis enfin intéressé à l'évolution temporelle d'un système à N-corps à partir d'un état non stationnaire. J'ai pu étudier la capacité de notre approche à reproduire la croissance linéaire de l’intrication dans le temps, ce qui est un obstacle fondamental pour les approches alternatives telles que le groupe de renormalisation de la matrice densitéThis thesis presents a study of quantum many-body systems at zero temperature, where the behavior of the system is purely driven by the quantum effects. I will introduce a variationnal approach developped with Tommaso Roscilde, my PhD supervisor, and Fabio Mezzacapo, my co-supervisor, in order to study these systems.This approach is based on a parametrisation of the quantum state (named Ansatz) on which we apply a variational optimisation, allowing us reproduce the system's evolution under Schrödinger's equation with a limited number of variables.By considering an imaginary-time evolution, it is possible to reconstruct the system's ground state. I focused on S=1/2 XX spin chain, where the long-range quantum correlations complicate a variational study; and I have specifically targeted our Ansatz in order to reproduce the correlations and the entanglement of the ground state. Moreover I considered the antiferromagnetic S=1/2 J1-J2 spin chain, where the non-trivial sign structure of the coefficients of the quantum state introduces an important challenge for the quantum Monte Carlo approach; and where the magnetic frustration induces a quantum phase transition (from a state with long range correlations to a non-magnetic state in the form of a valence-bond crystal).Finally I focused on the time evolution of a quantum many-body system starting from a non-stationary state. I studied the ability of our approach to reproduce the linear increase of the entanglement during time, which is a fondamental obstacle for other approaches such as the density-matrix renormalization group
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Brito, Lucas Menezes de. "Métodos variacionais aplicados à problemas singulares em equações elípticas não lineares." Universidade Federal de Goiás, 2018. http://repositorio.bc.ufg.br/tede/handle/tede/8860.
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Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - CAPES
In this work we study a singular partial differential problem in a bounded domain with smoth boundary. We have two main cases, one superlinear with weak singularity, and the other one sublinear with strong songularity. We use Variational Methods, such as the Ekeland Variational Principle and the Nehari Manifolds, to solve this problem, finding weak solutions and proving the multiplicity of solutions in one of the cases.
Neste trabalho estudaremos um problema diferencial parcial singular em um domínio limitado com bordo suave. Temos dois casos principais, um superlinear com singularidade fraca e um sublinear com singularidade forte. Usaremos Métodos Variacionais, como o Princípio Variacional de Ekeland e as Variedades de Nehari, para resolver este problema, encontrando soluções fracas e provando a multiplicidade das mesmas em um dos casos.
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ROMANI, GIULIO. "Positivity and qualitative properties of solutions of fourth-order elliptic equations." Doctoral thesis, Università degli Studi di Milano, 2017. http://hdl.handle.net/2434/525734.
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This thesis concerns the study of fourth-order elliptic boundary value problems and, in particular, qualitative properties of solutions. Such problems arise in various fields, from plate theory to conformal geometry and, compared to their second-order counterparts, they present intrinsic difficulties, mainly due to the lack of the maximum principle.
In the first part of the thesis, we study the positivity of solutions in case of Steklov boundary conditions, which are intermediate between Dirichlet and Navier boundary conditions. They naturally appear in the study of the minimizers of the Kirchhoff-Love functional, which represents the energy of a hinged thin and loaded plate in dependence of a parameter. We establish sufficient conditions on the domain to obtain the positivity of the minimizers of the functional. Then, for such domains, we study a generalized version of the functional. Using variational techniques, we investigate existence and positivity of the ground states, as well as their asymptotic behaviour for the relevant values of the parameter.
In the second part of the thesis we establish uniform a-priori bounds for a class of fourth-order semilinear problems in dimension 4 with exponential nonlinearities. We considered both Dirichlet and Navier boundary conditions and we suppose our nonlinearities positive and subcritical. Our arguments combine uniform estimates near the boundary and a blow-up analysis. Finally, by means of the degree theory, we obtain the existence of a positive solution.
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Saddek, Lhassane. "Solutions d'un problème aux limites non linéaire discontinu à l'infini." Paris 9, 1988. https://portail.bu.dauphine.fr/fileviewer/index.php?doc=1988PA090010.
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On recherche par une méthode variationnelle les solutions t-périodiques d'un système dynamique comportant un potentiel convexe sous quadratique ou super quadratique. On démontre des théorèmes d'existence d'une solution non triviale
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Zheng, Yuling. "Algorithmes bayésiens variationnels accélérés et applications aux problèmes inverses de grande taille." Thesis, Paris 11, 2014. http://www.theses.fr/2014PA112354/document.
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Dans le cadre de cette thèse, notre préoccupation principale est de développer des approches non supervisées permettant de résoudre des problèmes de grande taille le plus efficacement possible. Pour ce faire, nous avons considéré des approches bayésiennes qui permettent d'estimer conjointement les paramètres de la méthode avec l'objet d'intérêt. Dans ce cadre, la difficulté principale est que la loi a posteriori est en général complexe. Pour résoudre ce problème, nous nous sommes intéressés à l'approximation bayésienne variationnelle (BV) qui offre une approximation séparable de la loi a posteriori. Néanmoins, les méthodes d’approximation BV classiques souffrent d’une vitesse de convergence faible. La première contribution de cette thèse consiste à transposer les méthodes d'optimisation par sous-espace dans l'espace fonctionnel impliqué dans le cadre BV, ce qui nous permet de proposer une nouvelle méthode d'approximation BV. Nous avons montré l’efficacité de notre nouvelle méthode par les comparaisons avec les approches de l’état de l’art.Nous avons voulu ensuite confronter notre nouvelle méthodologie à des problèmes de traitement d'images de grande taille. De plus nous avons voulu favoriser les images régulières par morceau. Nous avons donc considéré un a priori de Variation Total (TV) et un autre a priori à variables cachées ressemblant à un mélange scalaire de gaussiennes par changement de positions. Avec ces deux modèles a priori, en appliquant notre méthode d’approximation BV, nous avons développé deux approches non-supervisées rapides et bien adaptées aux images régulières par morceau.En effet, les deux lois a priori introduites précédemment sont corrélées ce qui rend l'estimation des paramètres de méthode très compliquée : nous sommes souvent confronté à une fonction de partition non explicite. Pour contourner ce problème, nous avons considéré ensuite de travailler dans le domaine des ondelettes. Comme les coefficients d'ondelettes des images naturelles sont généralement parcimonieux, nous avons considéré des lois de la famille de mélange scalaire de gaussiennes par changement d'échelle (GSM) pour décrire la parcimonie. Une autre contribution est donc de développer une approche non-supervisée pour les lois de la famille GSM dont la densité est explicitement connue, en utilisant la méthode d'approximation BV proposéeIn this thesis, our main objective is to develop efficient unsupervised approaches for large dimensional problems. To do this, we consider Bayesian approaches, which allow us to jointly estimate regularization parameters and the object of interest. In this context, the main difficulty is that the posterior distribution is generally complex. To tackle this problem, we consider variational Bayesian (VB) approximation, which provides a separable approximation of the posterior distribution. Nevertheless, classical VB methods suffer from slow convergence speed. The first contribution of this thesis is to transpose the subspace optimization methods to the functional space involved in VB framework, which allows us to propose a new VB approximation method. We have shown the efficiency of the proposed method by comparisons with the state of the art approaches. Then we consider the application of our new methodology to large dimensional problems in image processing. Moreover, we are interested in piecewise smooth images. As a result, we have considered a Total Variation (TV) prior and a Gaussian location mixture-like hidden variable model. With these two priors, using our VB approximation method, we have developed two fast unsupervised approaches well adapted to piecewise smooth images.In fact, the priors introduced above are correlated which makes the estimation of regularization parameters very complicated: we often have a non-explicit partition function. To sidestep this problem, we have considered working in the wavelet domain. As the wavelet coefficients of natural images are generally sparse, we considered prior distributions of the Gaussian scale mixture family to enforce sparsity. Another contribution is therefore the development of an unsupervised approach for a prior distribution of the GSM family whose density is explicitly known, using the proposed VB approximation method
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DAMKE, Caíke da Rocha. "Problemas Elípticos Assintoticamente Lineares." Universidade Federal de Goiás, 2012. http://repositorio.bc.ufg.br/tede/handle/tde/1950.
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Previous issue date: 2012-02-02In this dissertation we analyze questions of existence and multiplicity of solutions for Dirichlet problem in the asymptotically linear case. To obtain our main results we use variational methods, such as Montain Pass Theorem and Linking Theorem.Moreover, we use the Liapunov-Schmidt reduction.
Nesta dissertação analisamos questões de existência e multiplicidade de soluções do problema de Dirichlet elíptico assintoticamente linear. Para obtermos os nossos principais resultados utilizamos métodos variacionais, tais como o Teorema do Passo da Montanha um Teorema de Linking. Além disso, utilizamos a redução de Liapunov-Schmidt.
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Pacquaut, Guillaume. "Couplage Stokes/Darcy dans un cadre Level-set en grandes déformations pour la simulation des procédés d'élaboration par infusion de résine." Phd thesis, Ecole Nationale Supérieure des Mines de Saint-Etienne, 2010. http://tel.archives-ouvertes.fr/tel-00609670.
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Ce travail de recherche propose un modèle numérique pour simuler les procédés par infusion de résine en utilisant la méthode des éléments finis. Ce modèle permet de représenter l'écoulement d'une résine liquide dans des préformes poreuses subissant de grandes déformations. Dans cette étude, une modélisation macroscopique est utilisée. Au niveau du procédé, une zone de résine liquide est déposée sur les préformes. Ces dernières étant considérées comme un milieu poreux. Les équations de Stokes et de Darcy sont utilisées pour modéliser l'écoulement de la résine respectivement dans le drainant et dans les préformes. L'originalité du modèle réside dans le fait qu'un seul maillage est utilisé pour les deux milieux. La discrétisation est réalisée avec des éléments mixtes : dans Stokes, des éléments P1+/P1 sont utilisés et dans Darcy, des éléments P1/P1 stabilisés avec une formulation multi-échelle sont employés. Des fonctions distances signées sont utilisées pour représenter l'interface entre Stokes-Darcy et pour représenter le front de résine. Concernant la déformation des préformes, une formulation Lagrangienne réactualisée est utilisée. Dans cette formulation Lagrangienne, le comportement des préformes humides est représenté à l'aide du modèle de Terzaghi dans lequel les préformes sèches ont un comportement élastique non-linéaire. La perméabilité est reliée à la porosité via la relation de Carman-Kozeny. Celle-ci est déterminée à partir de l'équation de conservation de la masse. Ce modèle a été implémenté dans ZéBuLoN. Plusieurs simulations numériques d'infusion de résine sont présentées à la fin de ce manuscrit.
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Santos, Flávia Milo dos. "Impacto hidrodinâmico vertical de corpos axissimétricos através de uma abordagem variacional." Universidade de São Paulo, 2013. http://www.teses.usp.br/teses/disponiveis/3/3152/tde-22092014-110644/.
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Do ponto de vista da hidrodinâmica clássica, o problema de impacto hidrodinâmico configura-se como um problema de contorno com fronteiras móveis cuja posição deve ser determinada simultaneamente à solução da equação de campo. Essa característica traz dificuldades para obtenção de soluções analíticas e numéricas. Nesse sentido, o presente trabalho propõe o desenvolvimento de um método numérico específico para analisar o problema de impacto hidrodinâmico de corpos sólidos rígidos contra a superfície livre da água. A solução da equação dinâmica não linear do problema de impacto depende da determinação do tensor de massa adicional a cada instante de tempo, o qual depende da posição e atitude do corpo no instante considerado. Um método variacional específico é empregado, através do qual os coeficientes de massa adicional são determinados com erro de segunda ordem, na posição considerada. Tal método é exemplo de técnicas numéricas dessingularizadas, através das quais o potencial de velocidade é aproximado em um espaço finito-dimensional formado por funções-teste derivadas de soluções potenciais elementares, tais como pólos, dipolos, anéis de dipolos, de vórtices, etc. O problema potencial de impacto hidrodinâmico, que se caracteriza pela dominância das forças de inércia, é formulado admitindo-se a superfície líquida como equipotencial, o que permite a analogia com o limite assintótico de frequência infinita do problema de radiação de ondas causada pelo movimento de corpos flutuantes. O método desenvolvido é então aplicado ao caso de impacto vertical de corpos axissimétricos, formulando o problema sob o chamado modelo de von Kármán generalizado (GvKM). Nesse modelo as condições de contorno na geometria exata do corpo são satisfeitas, porém os efeitos do empilhamento de água junto às raízes do jato, que se forma ao longo da intersecção com a superfície livre, não são considerados no caso geral. Resultados numéricos do coeficiente de massa adicional para uma família de esferoides são apresentados e tabulados para o pronto uso em análise e projeto. Além disso, considerações acerca da inclusão do efeito de empilhamento de água junto às raízes do jato, ou seja, da elevação da superfície livre são também feitas para o caso de esferas, fazendo uso de abordagens analíticas encontradas na literatura especializada.In terms of classical hydrodynamics, the hydrodynamic impact problem is characterized as a boundary problem with moving boundary which position must be determined simultaneously with the solution of the field equation. This feature brings difficulties to get analytical and numerical solutions. In this sense, the purpose of this work is to present a variational method technique specifically designed for the hydrodynamic impact problem of axisymmetric rigid bodies on the free surface. The solution of the nonlinear dynamic equation of the impacting motion depends on the determination of the added mass tensor and its derivative with respect to time at each integration time step. This is done through a variational method technique that leads to a second-order error approximation for the added mass if a first-order error approximation is sought for the velocity potential. This method is an example of desingularized numerical techniques, through which the velocity potential is approximated in a sub-space of finite dimension, formed by trial functions derived from elementary potential solutions, such as poles, dipoles, and vortex rings, which are placed inside the body. The potential problem of hydrodynamic impact, characterized by the dominance of inertial forces, is here formulated by assuming the liquid surface as equipotential, what allows the analogy with the infinity frequency limit in the usual free surface oscillating floating body problem. The method is applied to the vertical hydrodynamic impact of axisymmetric bodies within the so-called Generalized von Kármán Model (GvKM). In such approach, the exact body boundary condition is full-filled and the wet correction is not taken into account. Numerical results for the added mass coefficient for a family of spheroids are presented. Moreover, considerations are made on the effects of the free surface elevation for the specific case of an impacting sphere, through analytical approaches.
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Hanisch, Florian. "Variational problems on supermanifolds." Phd thesis, Universität Potsdam, 2011. http://opus.kobv.de/ubp/volltexte/2012/5975/.
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In this thesis, we discuss the formulation of variational problems on supermanifolds. Supermanifolds incorporate bosonic as well as fermionic degrees of freedom. Fermionic fields take values in the odd part of an appropriate Grassmann algebra and are thus showing an anticommutative behaviour. However, a systematic treatment of these Grassmann parameters requires a description of spaces as functors, e.g. from the category of Grassmann algberas into the category of sets (or topological spaces, manifolds). After an introduction to the general ideas of this approach, we use it to give a description of the resulting supermanifolds of fields/maps. We show that each map is uniquely characterized by a family of differential operators of appropriate order. Moreover, we demonstrate that each of this maps is uniquely characterized by its component fields, i.e. by the coefficients in a Taylor expansion w.r.t. the odd coordinates. In general, the component fields are only locally defined. We present a way how to circumvent this limitation. In fact, by enlarging the supermanifold in question, we show that it is possible to work with globally defined components.
We eventually use this formalism to study variational problems. More precisely, we study a super version of the geodesic and a generalization of harmonic maps to supermanifolds. Equations of motion are derived from an energy functional and we show how to decompose them into components. Finally, in special cases, we can prove the existence of critical points by reducing the problem to equations from ordinary geometric analysis. After solving these component equations, it is possible to show that their solutions give rise to critical points in the functor spaces of fields.In dieser Dissertation wird die Formulierung von Variationsproblemen auf Supermannigfaltigkeiten diskutiert. Supermannigfaltigkeiten enthalten sowohl bosonische als auch fermionische Freiheitsgrade. Fermionische Felder nehmen Werte im ungeraden Teil einer Grassmannalgebra an, sie antikommutieren deshalb untereinander. Eine systematische Behandlung dieser Grassmann-Parameter erfordert jedoch die Beschreibung von Räumen durch Funktoren, z.B. von der Kategorie der Grassmannalgebren in diejenige der Mengen (der topologischen Räume, Mannigfaltigkeiten, ...). Nach einer Einführung in das allgemeine Konzept dieses Zugangs verwenden wir es um eine Beschreibung der resultierenden Supermannigfaltigkeit der Felder bzw. Abbildungen anzugeben. Wir zeigen, dass jede Abbildung eindeutig durch eine Familie von Differentialoperatoren geeigneter Ordnung charakterisiert wird. Darüber hinaus beweisen wir, dass jede solche Abbildung eineindeutig durch ihre Komponentenfelder, d.h. durch die Koeffizienten einer Taylorentwickelung bzgl. von ungeraden Koordinaten bestimmt ist. Im Allgemeinen sind Komponentenfelder nur lokal definiert. Wir stellen einen Weg vor, der diese Einschränkung umgeht: Durch das Vergrößern der betreffenden Supermannigfaltigkeit ist es immer möglich, mit globalen Koordinaten zu arbeiten. Schließlich wenden wir diesen Formalismus an, um Variationsprobleme zu untersuchen, genauer betrachten wir eine super-Version der Geodäte und eine Verallgemeinerung von harmonischen Abbildungen auf Supermannigfaltigkeiten. Bewegungsgleichungen werden von Energiefunktionalen abgeleitet und wir zeigen, wie sie sich in Komponenten zerlegen lassen. Schließlich kann in Spezialfällen die Existenz von kritischen Punkten gezeigt werden, indem das Problem auf Gleichungen der gewöhnlichen geometrischen Analysis reduziert wird. Es kann dann gezeigt werden, dass die Lösungen dieser Gleichungen sich zu kritischen Punkten im betreffenden Funktor-Raum der Felder zusammensetzt.
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Bogfjellmo, Geir. "Discrete Invariant Variational Problems." Thesis, Norges teknisk-naturvitenskapelige universitet, Institutt for matematiske fag, 2011. http://urn.kb.se/resolve?urn=urn:nbn:no:ntnu:diva-13143.
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This thesis studies variational problems invariant under a Lie group transformation, and invariant discretizations of these. In chapters two and three, a general method for creating symplectic integrators preserving certain classes of variational symmetries of first order Lagrangians is developed and demonstrated. In chapters four and five, it is assumed that the discrete Lagrangian is invariant under a certain group action, and the Euler--Lagrange equations for the variational problem are expressed in the invariants of the group action.
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Gharsalli, Leila. "Approches bayésiennes en tomographie micro-ondes : applications à l'imagerie du cancer du sein." Thesis, Paris 11, 2015. http://www.theses.fr/2015PA112048/document.
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Ce travail concerne l'imagerie micro-onde en vue d'application à l'imagerie biomédicale. Cette technique d'imagerie a pour objectif de retrouver la distribution des propriétés diélectriques internes (permittivité diélectrique et conductivité) d'un objet inconnu illuminé par une onde interrogatrice connue à partir des mesures du champ électrique dit diffracté résultant de leur interaction. Un tel problème constitue un problème dit inverse par opposition au problème direct associé qui consiste à calculer le champ diffracté, l'onde interrogatrice et l'objet étant alors connus.La résolution du problème inverse nécessite la construction préalable du modèle direct associé. Celui-ci est ici basé sur une représentation intégrale de domaine des champs électriques donnant naissance à deux équations intégrales couplées dont les contreparties discrètes sont obtenues à l'aide de la méthode des moments. En ce qui concerne le problème inverse, hormis le fait que les équations physiques qui interviennent dans sa modélisation directe le rendent non-linéaire, il est également mathématiquement mal posé au sens de Hadamard, ce qui signifie que les conditions d'existence, d'unicité et de stabilité de la solution ne sont pas simultanément garanties. La résolution d'un tel problème nécessite sa régularisation préalable qui consiste généralement en l'introduction d'information a priori sur la solution recherchée. Cette résolution est effectuée, ici, dans un cadre probabiliste bayésien où l'on introduit une connaissance a priori adaptée à l'objet sous test et qui consiste à considérer ce dernier comme étant composé d'un nombre fini de matériaux homogènes distribués dans des régions compactes. Cet information est introduite par le biais d'un modèle de « Gauss-Markov-Potts ». De plus, le calcul bayésien nous donne la distribution a posteriori de toutes les inconnues connaissant l'a priori et l'objet. On s'attache ensuite à déterminer les estimateurs a posteriori via des méthodes d'approximation variationnelles et à reconstruire ainsi l'image de l'objet recherché. Les principales contributions de ce travail sont d'ordre méthodologique et algorithmique. Elles sont illustrées par une application de l'imagerie micro-onde à la détection du cancer du sein. Cette dernière constitue en soi un point très important et original de la thèse. En effet, la détection du cancer su sein en imagerie micro-onde est une alternative très intéressante à la mammographie par rayons X, mais n'en est encore qu'à un stade exploratoireThis work concerns the problem of microwave tomography for application to biomedical imaging. The aim is to retreive both permittivity and conductivity of an unknown object from measurements of the scattered field that results from its interaction with a known interrogating wave. Such a problem is said to be inverse opposed to the associated forward problem that consists in calculating the scattered field while the interrogating wave and the object are known. The resolution of the inverse problem requires the prior construction of the associated forward model. This latter is based on an integral representation of the electric field resulting in two coupled integral equations whose discrete counterparts are obtained by means of the method of moments.Regarding the inverse problem, in addition to the fact that the physical equations involved in the forward modeling make it nonlinear, it is also mathematically ill-posed in the sense of Hadamard, which means that the conditions of existence, uniqueness and stability of the solution are not simultaneously guaranteed. Hence, solving this problem requires its prior regularization which usually involves the introduction of a priori information on the sought solution. This resolution is done here in a Bayesian probabilistic framework where we introduced a priori knowledge appropriate to the sought object by considering it to be composed of a finite number of homogeneous materials distributed in compact and homogeneous regions. This information is introduced through a "Gauss-Markov-Potts" model. In addition, the Bayesian computation gives the posterior distribution of all the unknowns, knowing the a priori and the object. We proceed then to identify the posterior estimators via variational approximation methods and thereby to reconstruct the image of the desired object.The main contributions of this work are methodological and algorithmic. They are illustrated by an application of microwave imaging to breast cancer detection. The latter is in itself a very important and original aspect of the thesis. Indeed, the detection of breast cancer using microwave imaging is a very interesting alternative to X-ray mammography, but it is still at an exploratory stage
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Hofmann, Bernd, and Peter Mathé. "Parameter choice in Banach space regularization under variational inequalities." Universitätsbibliothek Chemnitz, 2012. http://nbn-resolving.de/urn:nbn:de:bsz:ch1-qucosa-86241.
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The authors study parameter choice strategies for Tikhonov regularization of nonlinear ill-posed problems in Banach spaces. The effectiveness of any parameter choice for obtaining convergence rates depend on the interplay of the solution smoothness and the nonlinearity structure, and it can be expressed concisely in terms of variational inequalities. Such inequalities are link conditions between the penalty term, the norm misfit and the corresponding error measure. The parameter choices under consideration include an a priori choice, the discrepancy principle as well as the Lepskii principle. For the convenience of the reader the authors review in an appendix a few instances where the validity of a variational inequality can be established.
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Cahill, Nathan D. "Constructing and solving variational image registration problems." Thesis, University of Oxford, 2009. http://ora.ox.ac.uk/objects/uuid:ed43a6f4-216f-45b5-88c5-2baaba1e684a.
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Nonrigid image registration has received much attention in the medical imaging and computer vision research communities, because it enables a wide variety of applications. Feature tracking, segmentation, classification, temporal image differencing, tumour growth estimation, and pharmacokinetic modeling are examples of the many tasks that are enhanced by the use of aligned imagery. Over the years, the medical imaging and computer vision communties have developed and refined image registration techniques in parallel, often based on similar assumptions or underlying paradigms. This thesis focuses on variational registration, which comprises a subset of nonrigid image registration. It is divided into chapters that are based on fundamental aspects of the variational registration problem: image dissimilarity measures, changing overlap regions, regularizers, and computational solution strategies. Key contributions include the development of local versions of standard dissimilarity measures, the handling of changing overlap regions in a manner that is insensitive to the amount of non-interesting background information, the combination of two standard taxonomies of regularizers, and the generalization of solution techniques based on Fourier methods and the Demons algorithm for use with many regularizers. To illustrate and validate the various contributions, two sets of example imagery are used: 3D CT, MR, and PET images of the brain as well as 3D CT images of lung cancer patients.
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Krömer, Stefan. "Nonconvex radially symmetric variational problems." Berlin Logos-Verl, 2005. http://deposit.ddb.de/cgi-bin/dokserv?id=2820900&prov=M&dok_var=1&dok_ext=htm.
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Liu, Yina. "Variational inequalities and optimization problems." Thesis, University of Liverpool, 2015. http://livrepository.liverpool.ac.uk/2031959/.
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The main purpose of this thesis is to study weakly sharp solutions of a variational inequality and its dual problem. Based on these, we present finite convergence algorithms for solving a variational inequality problem and its dual problem. We also construct the connection between variational inequalities and engineering problems. We consider a variational inequality problem on a nonempty closed convex subset of R^n. In order to solve this variational inequality problem, we construct the equivalence between the solution set of a variational inequality and optimization problems by using two gap functions, one is the primal gap function and the other is the dual gap function. We give properties of these two gap functions. We discuss su�cient conditions for the subdifferentiability of the primal gap function of a variational inequality problem. Moreover, we characterize relations between the G^ateaux differentiabilities of primal and dual gap functions. We also build some results for the Lipschitz and locally Lipschitz properties of primal and dual gap functions as well. Afterwards, several su�cient conditions for the relevant mapping to be constant on the solution set of a variational inequality has been obtained, including the relations between solution sets of a variational inequality and its dual problem as well as the optimal solution sets to gap functions. Based on these, we characterize weak sharpness of the solution set of a variational inequality by its primal gap function g and its dual gap function G. In particular, we apply error bounds of g, G and g + G on C. We also construct finite convergence of algorithms for solving a variational inequality by considering the convergence of a local projection. We carry out these results in terms of the weak sharpness of solution sets of a variational inequality as well as the error bounds of gap functions of a variational inequality problem.
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LEVI, FRANCESCA. "Variational principles for evolution problems." Doctoral thesis, Università degli studi di Brescia, 2023. https://hdl.handle.net/11379/571585.
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Lo studio di fenomeni che evolvono nel tempo è spesso condotto attraverso la loro modellazione come sistemi dinamici, la cui formulazione matematica, in genere, richiede la risoluzione di sistemi di equazioni differenziali a condizioni iniziali.
Risolvere le equazioni che governano un fenomeno fisico evolutivo significa determinarne l'evoluzione nel tempo a partire da un insieme di condizioni iniziali; ad esempio, considerando i sistemi meccanici, attraverso una legge matematica che ne determina la posizione e la velocità in funzione del tempo.
Tuttavia, le equazioni che governano il moto spesso non possono essere risolte analiticamente e quindi vengono utilizzate tecniche di integrazione numerica per ottenere un'approssimazione accurata della soluzione.
Trattare il problema dello studio di un sistema fisico da un punto di vista variazionale può essere un approccio diverso, motivato dalla formulazione Lagrangiana della meccanica classica.
L'idea di sostituire un dato problema con uno equivalente in forma variazionale non è certo nuova: l'interesse per questa formulazione è infatti giustificato dalla validità dei cosiddetti metodi diretti del calcolo delle variazioni.
Questi metodi sono validi sia per uno studio qualitativo del problema (verifica dell'esistenza e unicità della soluzione, la sua regolarità, ecc.), sia per uno studio quantitativo, cioè da
un punto di vista numerico (valutazione della convergenza, stima dell'errore della soluzione approssimata).
In questa tesi vengono analizzati problemi evolutivi di interesse ingegneristico, formulati per via variazionale.
In primo luogo, il problema viscoelastico lineare viene risolto numericamente utilizzando tre diverse formulazioni variazionali: la formulazione di Gurtin, la formulazione di Gurtin splittata e la formulazione di Huet.
Il metodo degli elementi finiti viene utilizzato per la discretizzazione spaziale e il metodo Ritz viene utilizzato per la discretizzazione temporale.
Successivamente, si prende in considerazione il problema della conduzione del calore. Vengono considerate due formulazioni: la prima basata su una forma bilineare convolutiva, la seconda su una forma bilineare biconvolutiva. Numerosi esperimenti numerici mettono in luce la bontà dei due diversi approcci.
Viene poi affrontato il tema della determinazione di upper e lower bounds per le proprietà meccaniche di materiali compositi costituiti da fasi aventi legami costitutivi viscoelastici.
Successivamente viene analizzato il problema dell'evoluzione di una frattura sia in un mezzo elastico sia in un mezzo viscoelastico. Nel primo caso viene proposta una formulazione estremale analoga a quella di Capurso e Maier, valida in ambito elastoplastico.
Infine, viene considerata la stabilità dinamica di sistemi piani con una sola massa concentrata e soggetti a forze follower.The study of phenomena that evolve over time is often conducted through their modelling as dynamic systems, whose mathematical formulation generally requires the resolution of systems of differential equations with initial conditions. Solving the governing equations of a physical phenomenon means determining its evolution over time starting from a set of initial conditions; for example, considering mechanical systems, through a mathematical law that determines its position and speed as functions of time. However, the equations governing motion cannot be often solved analytically and therefore, numerical integration techniques are used in order to obtain an accurate approximation of the solution. Treating the problem of studying a physical system from a variational point of view may be a different approach, motivated by the Lagrangian formulation of classical mechanics. The idea of replacing a given problem with an equivalent one in variational form is certainly not new: the interest in this formulation is in fact justified by the validity of the so-called direct methods of the calculation of variations. These methods are valid both for a qualitative study of the problem (verification of existence and uniqueness of the solution, its regularity, etc.), and for a quantitative study, namely from a numerical point of view (evaluation of convergence, estimation of the error of the approximate solution). In this thesis, evolution problems of engineering interest are analyzed, formulated in a variational way. Firstly, the linear viscoelastic problem is numerically solved using three different variational formulation, such as Gurtin's variational formulation, Split Gurtin formulation and the Huet formulation. The Finite Element Method is used for the space discretization and the Ritz method is used for the time discretization. Then, the heat conduction problem is taken into account. Two formulations are considered: the first one based on a convolutive bilinear form, the second one based on a biconvolutive bilinear form. Several numerical examples highlight the goodness of the two different approaches. Next, the problem of the determination of upper and lower bounds for the mechanical properties of composite materials, consisting of phases having viscoelastic constitutive laws, is addressed. Subsequently, the problem of the evolution of a fracture is analyzed both in an elastic medium and in a viscoelastic medium. In the first case, an extremal formulation, similar to that of Capurso and Maier, is proposed, valid in the elastoplastic field. Finally, the dynamical stability of plane systems with just one lumped mass, subjected to follower forces, is considered.
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Malavazi, Mazílio Coronel 1983. "Problemas elípticos do tipo côncavo-convexo com crescimento crítico e condição de Neumann." [s.n.], 2013. http://repositorio.unicamp.br/jspui/handle/REPOSIP/307128.
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Orientador: Francisco Odair Vieira de PaivaTese (doutorado) - Universidade Estadual de Campinas, Instituto de Matemática, Estatística e Computação Científica
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Doutorado
Matematica
Doutor em Matemática
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Hechaichi, Hadjer. "Problèmes de contrôle optimal associés avec des inégalités variationnelles et différentielles variationnelles." Thesis, Perpignan, 2019. http://www.theses.fr/2019PERP0008.
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Les problèmes de contrôle optimal se rencontrent dans l'industrie aérospatiale et dans la mécanique. Leur étude conduit à des difficultés mathématiques importantes. Dans cette thèse, nous nous intéressons aux conditions d'optimalité pour certains problèmes de contrôle avec des contraintes exprimées en termes d'inclusions différentielles. Nous considérons aussi des problèmes de contrôle associés aux modèles mathématiques issus de la Mécanique du Contact. Cette thèse est structurée en deux parties et six chapitres. La première partie, contenant les Chapitres 1, 2 et 3, représente un résumé de nos résultats, en Français. Nous y présentons les problèmes étudiés, les hypothèses sur les données, les notations utilisées ainsi que l’énoncé des principaux résultats. Les démonstrations sont omises. La deuxième partie du manuscrit représente la partie principale de la thèse. Elle contient les Chapitres 4, 5 and 6, chacun ayant fait l'objet d'une publication (parue ou soumise) dans une revue internationale avec comité de lecture.Nous y présentons nos principaux résultats, accompagnés des démonstrations et des références bibliographiquesOptimal control problems arise in aerospace industry and in mechanics. They are challenging and involve important mathematical difficulties. In this thesis, we are interested to derive optimality conditions for optimal control problems with constraints under the form of differential inclusions. We also consider optimal control problems in the study of some boundary value problems arising in Contact Mechanics. The thesis is structured in two parts and six chapters. Part I represents an abstract of the main results, in French. It contains Chapters 1, 2 and 3. Here we present the problems we study together with the assumptions on the data, the notation and the statement of the main results. The proofs of these results are omitted, since them are presented in Part II of the manuscript.Part II represents the main part of the thesis. It contains Chapters 4, 5 and 6. Each of these chapters made the object of a paper published (or submitted) in an international journal. Here we present our main results, together with the corresponding proofs and bibliographical references
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Iversen, Mette. "Variational problems : perturbations and optimal sets." Thesis, University of Bristol, 2012. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.619449.
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The calculus of variations looks in general at finding critical points of a given functional, and shape optimisation is specifically concerned with variational problems, where the unknown is the set on which the problem is defined. This thesis attempts to navigate through the topic of optimal sets, drawing on both the early beginnings of such investigations, and the work of the very active current research community. The behaviour under perturbations of quantities given by variational characterisations is a recurring theme. The starting point is a renewed look at the classical question of minimising individual eigenvalues of the Dirichlet. Laplacian over domains in Rn subject to geometric constraints, with particular focus on bounding the number of components of optimal sets. The results obtained give rise to questions concerning analogous results for minimisers of functions of eigenvalues. This thesis looks specifically at the corresponding properties for convex combinations of the first three eigenvalues, a study which in turn feeds back interesting information on the optimal sets for individual eigenvalues. Research on eigenvalue optimisation leads naturally to the study of the behaviour of the eigenvalues themselves and various other quantities under perturbations of a given domain. This thesis includes an investigation into bounds on the spectra of sets in terms of the principal frequency of their difference. Finally, a quantity of interest besides the eigenvalues is the torsional rigidity, and we conclude with an estimate of its behaviour under perturbations of a ball in Euclidean space Rn.
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Muller, Stefan. "Variational problems in mechanics and analysis." Thesis, Heriot-Watt University, 1989. http://hdl.handle.net/10399/925.
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