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Baldoni, Riccardo. "Investigation of spectral stability of X-ray tubes by simulations and experimental spectrum measurements." Master's thesis, Alma Mater Studiorum - Università di Bologna, 2016. http://amslaurea.unibo.it/10574/.
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L'obiettivo di questo lavoro è quello di analizzare la stabilità di uno spettro raggi X emesso da un tubo usurato per analisi cardiovascolari, in modo da verificare il suo comportamento. Successivamente questo tipo di analisi sarà effettuata su tubi CT. Per raggiungere questo scopo è stato assemblato un particolare set-up con un rivelatore al germanio criogenico in modo da avere la miglior risoluzione energetica possibile ed alcuni particolari collimatori così da ridurre il flusso fotonico per evitare effetti di pile-up. Il set-up è stato costruito in modo da avere il miglior allineamento possibile nel modo più veloce possibile, e con l'obiettivo di rendere l'intero sistema portabile. Il tubo usato è un SRM Philips tube per analisi cardiovascolari; questa scelta è stata fatta in modo da ridurre al minimo i fattori esterni (ottica elettromagnetica, emettitori) e concentrare l'attenzione solo sugli effetti, causati dalle varie esposizioni, sull'anodo (roughness e bending) e sul comportamento di essi durante il surriscaldamento e successivo raffreddamento del tubo. I risultati mostrano come durante un'esposizione alcuni fattori di usura del tubo possono influire in maniera sostanziale sullo spettro ottenuto e quindi alterare il risultato. Successivamente, nell'elaborato, mediante il software Philips di ricostruzione e simulazione dello spettro si è cercato di riprodurre, variando alcuni parametri, la differenza riscontrata sperimentalmente in modo da poter simulare l'instabilità e correggere i fattori che la causano. I risultati sono interessanti non solo per questo esperimento ma anche in ottica futura, per lo sviluppo di applicazioni come la spectral CT. Il passo successivo sarà quello di spostare l'attenzione su un CT tube e verificare se l'instabilità riscontrata in questo lavoro è persiste anche in una analisi più complessa come quella CT.
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Grünvogel, Stefan Michael. "Lyapunov spectrum and control sets." Augsburg [Germany] : Wissner-Verlag, 2000. http://catalog.hathitrust.org/api/volumes/oclc/45796984.html.
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Chrysler, Benjamin D., Pelaez Silvana Ayala, Yuechen Wu, Shelby D. Vorndran, and Raymond K. Kostuk. "Environmental stability study of holographic solar spectrum splitting materials." SPIE-INT SOC OPTICAL ENGINEERING, 2016. http://hdl.handle.net/10150/622712.
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In this study the impact of outdoor temperature variations and solar illumination exposure on spectral filter material and holographic optical elements is examined. Although holographic components have been shown to be useful for solar spectrum splitting designs, relatively little quantitative data exist to demonstrate the extent to which these materials can withstand outdoor conditions. As researchers seek to investigate practical spectrum splitting designs, the environmental stability of holographic materials should be considered as an important factor. In the experiment presented, two holographic materials, Covestro Bayfol HX photopolymer and dichromated gelatin, and 3M reflective polymer filter materials are exposed to outdoor conditions for a period of several months. The environmental effect on absorption, spectral and angular bandwidth, peak efficiency, and Bragg matching conditions for the holograms are examined. Spectral bandwidth and transmittance of the 3M reflective filter material are also monitored. Holographic gratings are recorded, measured, and mounted on glass substrates and then sealed with a glass cover plate. The test samples are then mounted on a photovoltaic panel to simulate realistic temperature conditions and placed at an outdoor test facility in Tucson, Arizona. A duplicate set of holograms and 3M filter material is stored as a control group and periodically compared over the test period.
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Krombach, Patricia A. "Effects of Stability Balls on Children with Autism Spectrum Disorder." Scholar Commons, 2016. http://scholarcommons.usf.edu/etd/6107.
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Children with ASD often display behavior problems that can lead to daily academic and social disruptions. Many teachers and therapists have sought to create classroom interventions that improve the length of time a child stays seated and focused on the required task. This has led to the introduction of stability balls as an alternative seating method for children, both on the autism spectrum and with other needs. This study used a multiple baseline design and duration data to evaluate the effects of stability ball seating on attending and in-seat behavior for children with ASD who received ABA therapy in their homes. The intervention replaced their standard seating method with a stability ball. In the final phase participants chose their own seating method before beginning table work to assess preference. Following intervention the stability ball was found to increase both attending and in-seat durations for children with ASD.
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Vronskaya, Maria. "Stability of singular spectrum analysis and causality in time series." Thesis, Cardiff University, 2013. http://orca.cf.ac.uk/52140/.
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The concept of causality has been widely studied in econometrics and statistics since 1969, when C. J. Granger published his paper "Investigating causal relations by econometric models and cross-spectral methods". The intuitive basis for his definition of causality is the following: time series Y is causing time series X if the use of the additional information provided by Y improves the forecast of series X. In the present thesis we focus on combining Granger's causality concept with the Singular Spectrum Analysis (SSA) technique. SSA is founded on the idea of transforming the time series into a multidimensional trajectory form (Hankel matrices), Singular Value Decomposition with subsequent projection to a lower-dimensional subspace and diagonal averaging. The main aim of the present thesis is to study the causality concept through SSA prism in details and suggest a novel causality measure, which can be used outside the stationary autoregressive class, which is the framework for Granger's original causality concept. We first apply standard statistical tests directly to simulated data to assess the improvement of forecast quality of bivariate multidimensional SSA (MSSA) of time series X and Y compared with SSA of time series X only. Although the results of performance of these tests are reasonably conclusive, the simulation method is time consuming and, thus, more theoretical understanding is desirable. We solve a fundamental scaling problem of the MSSA approach by introducing so-called linearized MSSA. The linearized MSSA approach shows a way towards a causality measure, calculated from the forecast linear recurrence formula (LRF) coefficients. We finally analyze SSA and (non-linear) bivariate MSSA approach in terms of first order stability analysis under perturbations leading to the construction of a valid suitable measure of causality. The construction of the measure requires some simplifying assumptions in the stability analysis whose validity we verify for both simulated and real data.
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MOLLOY, CYNTHIA ANN. "ASSESSMENT OF POSTURAL STABILITY IN CHILDREN WITH AUTISM SPECTRUM DISORDER." University of Cincinnati / OhioLINK, 2001. http://rave.ohiolink.edu/etdc/view?acc_num=ucin1006206215.
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Cramond, Alexander J. "The Longitudinal Stability of Memory in Males with Autism Spectrum Disorder." BYU ScholarsArchive, 2012. https://scholarsarchive.byu.edu/etd/3284.
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Previous research has demonstrated mixed evidence on impaired memory functioning in autism spectrum disorder (ASD), with the only consensus that there appears to be much heterogeneity. In addition, no research to date has examined the stability of memory in ASD. This study examined the stability of memory function in ASD compared to typically developing age-matched controls. Participants were administered the Test of Memory and Learning (TOMAL) twice, three years apart, in an established longitudinal NIH-supported investigation of ASD. Based on available research contrasting memory development in healthy individuals versus those with ASD, it was hypothesized that memory performance in the control group would be stable across time and that, compared to the control group, the autism group would demonstrate less stable memory function as measured by the TOMAL. Repeated Measures ANOVA and Reliable Change Index calculations of TOMAL Index and Subtest scores largely supported these hypotheses. The control group remained stable across time on all TOMAL indices and the ASD group showed improvement on the Composite Memory Index, Verbal Memory Index, and Delayed Memory Index but not the Non Verbal Memory Index. Clinical and research implications are discussed.
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Song, Degong. "On the Spectrum of Neutron Transport Equations with Reflecting Boundary Conditions." Diss., Virginia Tech, 2000. http://hdl.handle.net/10919/26375.
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This dissertation is devoted to investigating the time dependent neutron transport equations with reflecting boundary conditions. Two typical geometries --- slab geometry and spherical geometry --- are considered in the setting of L^p including L^1. Some aspects of the spectral properties of the transport operator A and the strongly continuous semigroup T(t) generated by A are studied. It is shown under fairly general assumptions that the accumulation points of { m Pas}(A):=sigma (A) cap { lambda :{ m Re}lambda > -lambda^{ast} }, if they exist, could only appear on the line { m Re}lambda =-lambda^{ast}, where lambda^{ast} is the essential infimum of the total collision frequency. The spectrum of T(t) outside the disk {lambda : |lambda| leq exp (-lambda^{ast} t)} consists of isolated eigenvalues of T(t) with finite algebraic multiplicity, and the accumulation points of sigma (T(t)) igcap{ lambda : |lambda| > exp (-lambda^{ast} t)}, if they exist, could only appear on the circle {lambda :|lambda| =exp (-lambda^{ast} t)}. Consequently, the asymptotic behavior of the time dependent solution is obtained.Ph. D.
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Feng, He. "A Study on Heterotic Target Space Duality – Bundle Stability/Holomorphy, F-theory and LG Spectra." Diss., Virginia Tech, 2019. http://hdl.handle.net/10919/102383.
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In the context of (0, 2) gauged linear sigma models, we explore chains of perturbatively dual heterotic string compactifications. The notion of target space duality (TSD) originates in non-geometric phases and can be used to generate distinct GLSMs with shared geometric phases leading to apparently identical target space theories. To date, this duality has largely been studied at the level of counting states in the effective theories. We extend this analysis in several ways.
First, we consider the correspondence including the effective potential and loci of enhanced symmetry in dual theories. By engineering vector bundles with non-trivial constraints arising from slope-stability (i.e. D-terms) and holomorphy (i.e. F-terms) the detailed structure of the vacuum space of the dual theories can be explored. Our results give new evidence that GLSM target space duality may provide important hints towards a more complete understanding of (0,2) string dualities. In addition, we consider TSD theories on elliptically fibered Calabi-Yau manifolds. In this context, each half of the "dual" heterotic theories must in turn have an F-theory dual. Moreover, the apparent relationship between two heterotic compactifications seen in (0,2) heterotic target space dual pairs should, in principle, induce some putative correspondence between the dual F-theory geometries. It has previously been conjectured in the literature that (0,2) target space duality might manifest in F-theory as multiple K3- fibrations of the same elliptically fibered Calabi-Yau manifold. In this work we investigate this conjecture in the context of both six-dimensional and four-dimensional effective theories and demonstrate that in general, (0,2) target space duality cannot be explained by such a simple phenomenon alone. Finally, we consider Landau-Ginzburg (LG) phases of TSD theories and explore their massless spectrum. In particular, we investigate TSD pairs involving geometric singularities. We study resolutions of these singularities and their relationship to the duality.Doctor of Philosophy
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Weldhagen, Gerhard Frederick. "Laboratory Detection and Gene Cassette Stability of the Novel Extended-Spectrum Beta-Lactamase, GES-2 from Pseudomonas aeruginosa." Thesis, University of Pretoria, 2004. http://hdl.handle.net/2263/29221.
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Extended-spectrum beta-lactamases (ESBLs) in Pseudomonas aeruginosa tend to be geographically scattered, such as GES-2, which partially compromises the efficacy of imipenem. The G170N mutation, ascribed to a CC to AA base pair substitution on positions 493-494 of the blaGES-2 coding region, distinguishes this ESBL from blaGES-1 and the blaIBC-type genes, making it an ideal target for developing a novel sequence-specific, peptide nucleic acid (PNA)-based, multiplex-PCR detection method. Utilizing two primer pairs in conjunction with a PNA probe, this novel method delivered accurate identification of blaGES-2 compared to standard PCR and gene sequencing techniques, when tested against one hundred (n = 100) P. aeruginosa clinical isolates as well as previously published, well-described control strains. This method has the potential to be used in large-scale, cost-effective screening programmes for specific or geographically restricted ESBLs. To date, in addition to being only described in South Africa, GES-2 is notoriously difficult to identify in P. aeruginosa, using standard methodology. A real-time PCR method using the LightCycler™ was compared to a two-step nested-PCR assay for the detection of blaGES and blaIBC genes from one hundred P. aeruginosa clinical isolates collected over a four-year period from two teaching hospitals in Pretoria, South Africa. Real-time PCR amplification was monitored through hybridisation of fluorescently labelled probes followed by melting curve analysis to detect the relevant G170N mutation occurring in the omega loop region of blaGES-2. Nested-PCR products were subjected to automated DNA sequencing and compared to melting point (Tm) analyses results obtained from the LightCycler assay. Real time and nested-PCR assays detected a blaIBC gene product from 83 and 88 clinical isolates respectively, with the LightCycler thus exhibiting a sensitivity of 94.3% compared to the nested-PCR assay. Comparison of Tm and gene sequencing data however revealed 100% specificity for sequence specific detection of blaGES-2 with the LightCycler. One clinical isolate was found to harbour a blaGES-1 gene, making this the first report of this specific ESBL from South Africa. Selective antibiotic pressure has recently been implicated as a possible driving force behind point mutations observed in blaGES–type genes. This part of the study subjected two well-characterized clinical isolates with class 1 integron-borne blaGES-type genes to five days incubation in the presence of sub-inhibitory concentrations of 15 different antibiotics, including beta-lactams, aminoglycosides and quinolones. Restriction enzyme analysis and DNA sequencing of blaGES-1, blaGES-2 and their immediate upstream genetic environments failed to demonstrate any changes compared to non-exposed controls. Short-term exposure to a sub-inhibitory level of a single antimicrobial agent is thus unlikely to select significant mutations in these beta-lactamase genes or their regulatory mechanisms.Thesis (PhD (Medical Microbiology))--University of Pretoria, 2006.
Medical Microbiology
unrestricted
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Smoot, Senia I. "Effect of an Acute Sensory Integration Therapy on the Postural Stability and Gaze Patterns of Children with Autism Spectrum Disorder." University of Dayton / OhioLINK, 2013. http://rave.ohiolink.edu/etdc/view?acc_num=dayton1386151416.
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Benoit, Valérie. "La brévicine 27, une bactériocine produite par Lactobacillus brevis SB27 : caractérisation, purification et éléments de structure chimique." Vandoeuvre-les-Nancy, INPL, 1996. http://www.theses.fr/1996INPL101N.
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Lactobacillus brevis SB27, souche isolée à partir de saucisson, produit un agent antibactérien actif contre les lactobacilles hétérofermentaires (Lb brevis et Lb büchneri), des pédiocoques et des souches de Bacillus. Cet agent bactéricide est de nature protéique et présente un spectre d'action centré sur les espèces phylogénétiquement proches du microorganisme producteur. Il a donc été identifié à une bactériocine, qui fut nommée brévicine 27. Son activité antibactérienne est très résistante aux traitements thermiques, surtout à pH acide. Des expériences d'ultrafiltration, de filtration sur gel et d'électrophorèse SDS-PAGE (couplée à une révélation biologique de l'activité) ont montré qu'en conditions natives la bactériocine formait des agrégats dont les composants pouvaient être dissociés en conditions dénaturantes. Les sous-unités actives ont une masse moléculaire apparente comprise entre 2,5 et 5 kDa. Des cures plasmidiques à la novobiocine ont permis d'obtenir un variant de Lb brevis SB27, apparemment non-producteur de bactériocine et sensible à la bactériocine parentale. L’analyse des profils plasmidiques a montré la perte d'une bande plasmidique de 3 MDa pour ce variant. La purification de la brévicine 27 a été effectuée à partir du surnageant de culture de Lb brevis SB27, prélevé en début de phase stationnaire. Le protocole de purification combine une précipitation au sulfate d'ammonium, des chromatographies d'échanges de cations et d'interactions hydrophobes et une chromatographie liquide haute performance en phase inverse. La bactériocine purifiée a été soumise à des analyses de spectrométrie de masse, de composition en acides aminés et de séquençage par dégradation d'Edman. La brévicine 27 a une masse moléculaire moyenne de 5202,3 Da. Elle comprend de nombreux résidus lysine et une forte proportion d'acides aminés hydrophobes. L’identification de 25 résidus sur les 28 résidus N-terminaux montre l'originalité de la brévicine 27
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Singh, Navrag B. "Functional competency of lower limb musculature in the elderly." Doctoral thesis, Humboldt-Universität zu Berlin, Philosophische Fakultät IV, 2013. http://dx.doi.org/10.18452/16772.
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Körperlich aktiv zu sein ist Grundlage unseres täglichen Lebens. Für alle diese Aktivitäten ist das kontinuierliche Zusammenspiel des senso-motorischen System (SMS) erforderlich. Die Kontrolle der verschiedenen afferenten und efferenten Subsysteme innerhalb des SMS basiert auf Feedback-Mechanismen, die die Aufrechterhaltung des Gleichgewichts und der Stabilität während den verschiedensten statischen als auch dynamischen Aktivitäten ermöglichen. Trotz dieser Kontroll- und Stabilisierungssystems ist das kinematische und kinetische Resultat nicht konstant; stattdessen ist bei globalen „Ganzkörper-Bewegungen“, und lokaler Muskelanspannung ständig eine gewisse Variabilität vorhanden. Die Interpretation dieser Variabilität bei Bewegungshandlungen ist kontrovers. Wobei große Variabilität ist nicht zwangsläufig ein Indikator für Defizite des SMS darstellt. Das Ziel dieser Dissertation war, die Variabilität bei lokalen und globalen Bewegungshandlungen in statischen und dynamischen Ausgangstellungen zu quantifizieren. Darüberhinaus, wurde der Zusammenhang zwischen lokaler Variabilität der Muskelkraftproduktion und der Variabilität bei globalen Bewegungshandlungen. Die Ergebnisse zeigen, dass lokale und globale Variabilität von Bewegungshandlungen in Menge und Muster verändert sind, nach Störung des SMS durch: Ermüdung, Veränderungen der Umfeldbedingungen, Alterung und bei Personen mit Sturzerfahrung. Außerdem wurde gezeigt, dass sowohl zu große als auch zu kleine Variabilität, ein entscheidendes funktionelles Defizit bei älteren Personen darstellt. Dieser Dissertation hebt die Bedeutung der Variabilität während wiederholter Bewegungshandlungen hervor, welche einen funktionellen Biomarker für die Beurteilung von Bewegungsstörungen darstellt. In der klinische Praxis könnte dieser helfen bei der frühen Identifikation von Personen mit Bewegungsstörungen, zur Entwicklung von individual-spezifischen Rehabilitationsmaßnahmen, sowie der Beurteilung verschiedener Therapieansätze.Undertaking activities is fundamental throughout daily living. In order to successfully perform these activities, continuous involvement of the human sensori-motor system (HSMS) is required. The HSMS involves feedback mechanisms to control numerous afferent and the efferent subsystems to ensure maintenance of balance and stability during both static and dynamic activities. Despite such control and stabilizing mechanisms, the kinematic and kinetic output of a task is not constant; instead variability occurs during continuous performance of both global tasks such as standing and walking, as well as local force production. The interpretation of variability during output task performance remains controversial, with larger levels of variability not always indicating deficits in human-motor performance. The aim of this dissertation was to assess variability during local as well as global task performance in static and dynamic settings. Furthermore, the association between the level of variability during local force production and variability during global tasks such as standing and walking was also investigated. The results within this dissertation showed that variability during task performance is modified in magnitude as well as in structure after perturbation due to fatigue, changes in environmental conditions, and aging, as well as in fall-prone elderly individuals. Furthermore, both high as well as low levels of variation constitute a key functional deficit among elderly individuals. This dissertation highlights the importance of considering trial-to-trial variations during continuous task performance as a key functional biomarker for motor-related pathologies. Effective assessment of such measures of variability in clinical settings could effectively complement current clinical practice for both early and effective identification of individuals with motor-related pathology, designing subject-specific rehabilitation programs, and evaluating therapy efficacy.
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Al, Sheikh Lamis. "Scattering resonances and Pseudospectrum : stability and completeness aspects in optical and gravitational systems." Electronic Thesis or Diss., Bourgogne Franche-Comté, 2022. http://www.theses.fr/2022UBFCK007.
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Le contexte général de cette thèse est un effort pour établir un pont entre la physique gravitationnelle et optique, spécifiquement dans le contexte des problèmes de diffusion à l’aide des concepts et des outils tirés de la théorie des opérateurs non auto-adjoints. Nous nous concentrons sur les modes quasi-normaux (MQN), appelés les modes de résonance naturels des structures à fuites ouvertes sous des perturbations linéaires soumises à des conditions de bords sortantes.Ils sont également appelés résonances de diffusion. Dans le cas auto-adjoint conservateur, le théorème spectral garantit la complétude et la stabilité spectrale des modes normaux associés.En ce sens, une question naturelle dans le cadre de non auto-adjoint est reliée à la caractérisation et à l’évaluation des notions appropriées de complétude de MQNs et de stabilité spectrale dans les systèmes ouverts non conservateurs. Ceci définit les objectifs de cette thèse. Pour ce faire, et contrairement à l’approche traditionnelle des résonances de diffusion, nous adoptons une méthodologie dans laquelle les MQNs sont présentés comme un problème spectral d’un opérateur approprié non auto-adjoint. Plus précisément, cette méthodologie est basée sur les trois ingrédients suivants :(i) L’approche hyperboloïdale: L’approche en tranchant hyperboloïdales est déjà utilisée dansles problèmes gravitationnels, nous l’avons introduite dans les problèmes optiques. L’idéeest d’étudier l’équation d’onde en tranches hyperboliques au lieu des tranches de Cauchy habituellement utilisées. Le système de coordonnées est plus adapté à la problématique des QNMs et de ses conditions aux limites sortantes, en particulier, aborder les modes explosifs dans l’approche de Cauchy. Les modes sont normalisables en de telles coordonnées ettravailler dans ces tranches éliminent le besoin d’imposer les conditions de bords sortantes.(ii) Pseudospectre d’un opérateur: la notion de epsilon-pseudospectre permet d’évaluer la (in)stabilité des valeurs propres d’un opérateur dans le plan complexe en raison d’une perturbation de l’opérateur d’ordre epsilon. Cette thèse introduit la notion de pseudospectre en physique gravitationnel et optique au voisinage des valeurs propres.(iii) Au niveau technique, les méthodes spectrales fournissent un outil efficace pour traduirele problème en un problème numérique. En particulier, nous avons utilisé la base de Chebyshev pour l’expansion des nos champs. Les résultats de ce travail touchent trois domaines :(i) L’instabilité des MQN pour certaines classes de potentiels. Les modes fondamentaux sont stables spécialement sous de petites perturbations "à haute fréquence", alors que les harmoniques sont sensibles à de telles perturbations. L’instabilité des harmoniques augmente à mesure que leur partie imaginaire grandit.(ii) L’universalité du comportement asymptotique des MQNs et du pseudospectre. Nous remarquons un comportement asymptotiquement logarithmique des lignes de contour du pseudospectre et délimitant les branches d’ouverture des MQNs par le bas.(iii) MQNs expansion. Nous revisitons les expansions résonantes asymptotiques de Lax &Phillips d’un "champ diffusé" en termes de MQNs pour nos problèmes physiques. En particulier, nous utilisons le développement de Keldysh des généralisations des expressions pour les modes normaux des systèmes conservateurs, spécifiquement en termes de fonctions propres MQN normalisables et d’expressions explicites pour les coefficients d’excitationThe general context of this thesis is an effort to establish a bridge between gravitational andoptical physics, specifically in the context of scattering problems using as a guideline concepts andtools taken from the theory of non-self-adjoint operators. Our focus is on Quasi-Normal Modes(QNMs), namely the natural resonant modes of open leaky structures under linear perturbationssubject to outgoing boundary conditions. They also are referred to as scattering resonances.In the conservative self-adjoint case the spectral theorem guarantees the completeness andspectral stability of the associated normal modes. In this sense, a natural question in the non-self-adjoint setting refers to the characterization and assessment of appropriate notions of QNMcompleteness and spectral stability in open non-conservative systems. This defines the generalobjective of this thesis. To this aim, and in contrast with the traditional approach to scatter-ing resonances, we adopt a methodology in which QNMs are cast as a spectral problem of anappropriate non-self-adjoint operator. Specifically this methodology is based on following threeingredients:(i) Hyperboloidal approach: The hyperboloidal slicing approach is already used in gravitationalproblems, we introduced it here to optical ones. The idea is to study the wave equationin hyperbolic slices instead of usually used Cauchy slices. The system of coordinates ismore adapted to the problem of QNMs and its outgoing boundary conditions, in particularaddressing the exploding modes in the Cauchy approach. The modes are normalizable insuch coordinates and working in these slices eliminate the need of imposing the outgoingboundary conditions.(ii) Pseudospectrum of an operator: the notion of epsilon-pseudospectrum allows to assess the (in)stabilityof eigenvalues of an operator in the complex plane due to a perturbation to the operator oforder epsilon. This thesis introduces the notion of pseudospectrum in gravitational and opticalphysics in the vicinity of the eigenvalues.(iii) Numerical Chebyshev spectral methods: On the technical level, spectral methods providesan efficient tool when translating the problem into a numerical one. In particular we usedChebyshev basis to expand our fields.The results of this work touch three areas:(i) The instability of QNMs for some class of potentials. The fundamental modes are stablespecially under small "high frequency" perturbations, whereas overtones are sensitive tosuch perturbations. The instability of the overtones increases as their imaginary part grows.(ii) The universality of the asymptotic behaviour of QNMs and pseudospectrum. We remarkan asymptotically logarithmic behavior of pseudospectrum contour lines and bounding theopening QNMs branches from below.(iii) QNMs expansion. We revisit Lax & Phillips asymptotic resonant expansions of a "scattered field" in terms of QNMs in our physical settings. In particular , we make use of Keldysh expansion of the generalizations of the expressions for normal modes of conservative systems, specifically in terms of normalizable QNM eigenfunctions and explicit expressions for the excitation coefficients
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Sauvage, Jean-Paul, and Jean-François Verchère. "Relations structure-stabilité dans les complexes molybdiques et tungstiques des glucides." Rouen, 1993. http://www.theses.fr/1993ROUES030.
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En milieu acide, les glucides (sucres et alditols) forment des complexes anioniques avec les ions molybdate et tungstate. Les constantes de formation de ces complexes ont été déterminées par une méthode potentiométrique originale qui a permis, par ailleurs, de rationaliser le dosage acidimétrique des tungstates. La stabilité de ces complexes dépend de leur structure qui est imposée par la configuration des hydroxyles du ligand. On peut donc espérer différencier des glucides isomères, en les engageant dans des combinaisons chargées, réversibles, de stabilités différentes. L'étude structurale a été réalisée par RMN. Elle a permis l'identification de sites de chélation tri-, tétra- et pentadentés adaptés aux groupes dimétalliques, faisant intervenir le ligand sous forme acyclique ou hémiacétalique (pyranose ou furanose). Selon la configuration érythro ou thréo du site de chélation, on a observé un comportement différent des anions tungstate et molybdate, ce qui est extrêmement rare dans la chimie de ces deux éléments. L'interprétation thermodynamique des différences de stabilité entre les complexes acycliques de sucres et ceux d'alditols, a conduit à la détermination des proportions de forme ouverte en solution aqueuse, pour certains aldoses
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Khan, Mustayeen Ahmed. "Stabilite, spectres electroniques et structures des halogenocuprates en milieu protique." Université Louis Pasteur (Strasbourg) (1971-2008), 1988. http://www.theses.fr/1988STR13059.
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Etude des complexes de cu(ii) dans les solvants protiques l'eau, le methanol et l'acide acetique. On caracterise 4 complexes successsifs; calcul de leurs constantes de stabilite. Etude de l'effet du solvant sur les spectres electroniques
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Parveen, Tania. "Composite load model decomposition : induction motor contribution." Thesis, Queensland University of Technology, 2009. https://eprints.qut.edu.au/31301/1/Tania_Parveen_Thesis.pdf.
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In this thesis, a new technique has been developed for determining the composition of a collection of loads including induction motors. The application would be to provide a representation of the dynamic electrical load of Brisbane so that the ability of the power system to survive a given fault can be predicted. Most of the work on load modelling to date has been on post disturbance analysis, not on continuous on-line models for loads. The post disturbance methods are unsuitable for load modelling where the aim is to determine the control action or a safety margin for a specific disturbance. This thesis is based on on-line load models. Dr. Tania Parveen considers 10 induction motors with different power ratings, inertia and torque damping constants to validate the approach, and their composite models are developed with different percentage contributions for each motor. This thesis also shows how measurements of a composite load respond to normal power system variations and this information can be used to continuously decompose the load continuously and to characterize regarding the load into different sizes and amounts of motor loads.
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Schwartz, Jean-Luc. "Représentations auditives de spectres vocaliques." Grenoble INPG, 1987. http://www.theses.fr/1987INPG0086.
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Ce travail se compose en trois parties. La premiere partie est consacree aux techniques d'estimation par masquage des caracteristiques quantitatives des representations auditives "peripheriques". La deuxieme partie traite de l'etude des traitements auditifs des spectres vocaliques. Dans la troisieme partie les auteurs abordent l'etude des systemes vocaliques en proposant ce que pourrait etre une "theorie des formes et de la stabilite" a la lumiere de la theorie de l'integration spectrale e large bande, avec les concepts de stabilite intrinseques et extrinseques, et en essayant d'inscrire cette theorie dans un systeme general des systemes vocaliques
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Ferraresso, Francesco. "On the spectral stability of polyharmonic operators on singularly perturbed domains." Doctoral thesis, Università degli studi di Padova, 2018. http://hdl.handle.net/11577/3426790.
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In this thesis, we analyse the spectral convergence properties of higher order elliptic differential operators subject to singular domain perturbations and non-Dirichlet boundary conditions, with special attention to polyharmonic operators. We identify suitable conditions on the shape of the initial domain, on the shape of the perturbed domains, and on the geometry of the perturbation in order to assure the spectral stability. We find the limiting differential problem depending on the type of domain perturbation and the geometrical parameters governing the shape deformation of the initial domain. We prove that, under suitable conditions, the eigenvalues and the eigenprojections of the given differential operator in the perturbed domain converge to the eigenvalues and the eigenprojections of the limiting differential operator in the unperturbed domain. Finally, we prove convergence of the resolvent operators in the framework of the compact convergence of linear operators in Hilbert spaces.
More specifically, we first analyse the spectral convergence of a family of higher order self-adjoint elliptic operators subject to intermediate boundary conditions on perturbed domains defined locally by the hypographs of given functions. We prove a spectral stability theorem for this family of operators under the assumption that the convergence of the functions describing the boundary of the domain is sufficiently regular. Then we apply the theorem to study the spectral behaviour of polyharmonic operators with intermediate boundary conditions when the boundary of the domain undergoes a perturbation of oscillatory type, by adapting techniques introduced by J.M. Arrieta and P.D. Lamberti for the biharmonic operator. We prove that the limiting differential problem depends on the ratio between the amplitude and the period of the oscillation. Indeed there is a critical threshold above which there is spectral stability; that is, the eigenvalues and the eigenprojections of the perturbed problem converge to the corresponding eigenvalues and eigenprojections of the same differential problem in the limiting domain. Instead, under that threshold there is a different behaviour depending on the order of the polyharmonic operator and on the type of intermediate boundary conditions imposed at the boundary. If the ratio assumes exactly the critical value, then the limiting differential problem exhibits a strange boundary condition, which is characterized in terms of an auxiliary function satisfying a suitable differential problem. In order to treat this critical case we use homogenization techniques and macroscopic-microscopic decompositions, inspired by arguments used by J. Casado-Diaz and collaborators.
Then we consider the biharmonic operator and the Reissner-Mindlin operator subject to homogeneous boundary conditions of Neumann type on a planar dumbbell domain which consists of two disjoint domains connected by a thin channel. We analyse the spectral behaviour of the operator, characterizing the limit of the eigenvalues and of the eigenprojections as the thickness of the channel goes to zero, in the spirit of the articles by J.M. Arrieta and collaborators for the Neumann Laplacian. In applications to linear elasticity, the operators under consideration are related to the deformation of a free elastic plate, a part of which shrinks to a segment. In contrast to the classical case of the Laplace operator, it turns out that the limiting equation is here distorted by a strange factor depending on a parameter which plays the role of the Poisson coefficient of the represented plate.In questa tesi si studia la dipendenza degli autovalori di operatori differenziali ellittici di ordine superiore da perturbazioni singolari del dominio, con attenzione per gli operatori poliarmonici e per condizioni al bordo di tipo intermedio e Neumann. Si identificano opportune condizioni geometriche sul dominio iniziale, sui domini perturbati e sulla perturbazione al fine di assicurare la stabilità spettrale. Si caratterizzano i problemi differenziali limite, al variare dei parametri che regolano la deformazione del dominio iniziale. Si dimostra che, assumendo opportune ipotesi, gli autovalori e le proiezioni sugli autospazi associati al problema differenziale nel dominio perturbato convergono ai rispettivi autovalori e proiezioni associati al problema limite nel dominio iniziale. Inoltre si dimostra che i risolventi convergono compattamente al risolvente associato al problema limite. In particolare, si analizza dapprima la convergenza spettrale di una famiglia di operatori autoaggiunti, ellittici, di ordine superiore, con condizioni al bordo di tipo intermedio, su domini perturbati definiti localmente dal sottografico di date funzioni. Si dimostra un teorema di stabilità spettrale assumendo che la convergenza delle funzioni che rappresentano localmente la frontiera convergano in modo sufficientemente regolare. Si utilizza poi tale risultato per studiare il comportamento spettrale di operatori poliarmonici con condizioni al bordo di tipo intermedio quando la frontiera del dominio è soggetta ad una oscillazione periodica e singolare, adattando delle tecniche utilizzate da J.M. Arrieta e P.D. Lamberti nel caso dell'operatore biarmonico. Si dimostra che il problema limite dipende dal rapporto tra l'ampiezza dell'oscillazione e il periodo di oscillazione. Infatti esiste un valore limite per questo rapporto al di sopra del quale si ha stabilità spettrale, cioè gli autovalori e le proiezioni sugli autospazi associati alla famiglia di domini perturbati convergono ai corrispondenti autovalori e proiezioni associati allo stesso operatore differenziale nel dominio limite; al di sotto di tale valore critico invece l'operatore differenziale limite è differente, in quanto assume condizioni al bordo diverse sulla frontiera del dominio limite. Infine se il rapporto assume esattamente il valore critico, appare un `termine strano' in una delle condizioni al bordo associate al problema limite, che è stato caratterizzato in funzione della soluzione di un dato problema al bordo ausiliario. In questo caso limite si sfruttano tecniche dimostrative tipiche dell'omogeneizzazione periodica, come il metodo di `unfolding' e le decomposizioni micro-macroscopiche delle funzioni di Sobolev, presenti, ad esempio, in alcuni articoli di J. Casado-Diaz e collaboratori. Nel piano euclideo si considerano inoltre l'operatore biarmonico e l'operatore associato al sistema di Reissner-Mindlin, con condizioni al bordo di tipo Neumann, su un dominio `a bilanciere', che consiste di due domini regolari, limitati e disgiunti, collegati attraverso un canale sottile. Si analizza il comportamento limite dello spettro degli operatori e si caratterizza il limite degli autovalori e delle proiezioni sugli autospazi quando la larghezza del canale diminuisce fino ad annullarsi, adattando tecniche introdotte da J.M. Arrieta e collaboratori per l'operatore di Laplace con condizioni al bordo di tipo Neumann. Nelle applicazioni alla teoria dell'elasticità lineare, gli operatori in considerazione sono collegati alla deformazione di una piastra elastica, di materiale omogeneo e non vincolata, dovuta alla degenerazione di una delle sue dimensioni. In contrasto con il caso dell'operatore di Laplace, l'equazione limite risulta distorta da un coefficiente strano, che dipende dal coefficiente di Poisson della piastra modellizzata.
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Novickij, Jurij. "Hiperbolinės lygties su nelokaliosiomis kraštinėmis sąlygomis skirtuminio sprendinio stabilumas." Master's thesis, Lithuanian Academic Libraries Network (LABT), 2014. http://vddb.library.lt/obj/LT-eLABa-0001:E.02~2012~D_20140704_175149-44355.
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Darbo tikslas — ištirti baigtiniu skirtumu metodo antrosios eiles hiperbolinio tipo diferencialinei lygciai su nelokaliosiomis integralinemis kraštinemis salygomis stabiluma. Siekiant numatyto tikslo buvo sprendžiami šie uždaviniai: • išnagrinetas antrosios eiles hiperbolines lygties trisluoksnes skirtumines schemos suvedimas i dvisluoksne skirtumine schema; • išanalizuotas skirtuminio operatoriaus perejimo matricos spektras; • gauta pakankamoji skirtumines schemos stabilumo salyga, nusakoma nelokaliuju salygu parametrais; • atlikti skaitiniai eksperimentai, patvirtinantys teorines išvadas. Nurodyta stabilumo salyga yra esmine, sprendžiant hiperbolinio tipo uždavinius su pakankamai didelemis T reikšmemis. Skirtuminio operatoriaus perejimo matricos spektro tyrimo metodika gali buti pritaikyta placios klases diferencialiniu lygciu su nelokaliosiomis salygomis stabilumui tirti.On the stability of an explicit difference scheme for hyperbolic equation with integral conditions. The aim of the work is stability analysis of solution of finite difference method for hyperbolic equations. Trying to achieve formulated aim these tasks were solved: • a method of transformation of three-layered finite difference scheme into two-layered one was investigated; • a spectrum of transition matrix subject to the properties of second order differential operator Lambda was studied; • stability conditions of hyperbolic type equations with nonlocal conditions subject to boundary parameters were obtained; • numerical experiments, confirming theoretical derivations were made. Derived results could be used to solve one-dimensional tasks with hyperbolic equations in different sciences, to analyse spectrum structure of mathematical models and construct new numerical methods for solving hyperbolic PDEs.
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Akil, Mohammad. "Quelques problèmes de stabilisation directe et indirecte d’équations d’ondes par des contrôles de type fractionnaire frontière ou de type Kelvin-Voight localisé." Thesis, Limoges, 2017. http://www.theses.fr/2017LIMO0043/document.
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Cette thèse est consacrée à l’étude de la stabilisation directe et indirecte de différents systèmes d’équations d’ondes avec un contrôle frontière de type fractionnaire ou un contrôle local viscoélastique de type Kelvin-Voight. Nous considérons, d’abord, la stabilisation de l’équation d’ondes multidimensionnel avec un contrôle frontière fractionnaire au sens de Caputo. Sous des conditions géométriques optimales, nous établissons un taux de décroissance polynomial de l’énergie de système. Ensuite, nous nous intéressons à l’étude de la stabilisation d’un système de deux équations d’ondes couplées via les termes de vitesses, dont une seulement est amortie avec contrôle frontière de type fractionnaire au sens de Caputo. Nous montrons différents résultats de stabilités dans le cas 1-d et N-d. Finalement, nous étudions la stabilité d’un système de deux équations d’ondes couplées avec un seul amortissement viscoélastique localement distribué de type Kelvin-VoightThis thesis is devoted to study the stabilization of the system of waves equations with one boundary fractional damping acting on apart of the boundary of the domain and the stabilization of a system of waves equations with locally viscoelastic damping of Kelvin-Voight type. First, we study the stability of the multidimensional wave equation with boundary fractional damping acting on a part of the boundary of the domain. Second, we study the stability of the system of coupled onedimensional wave equation with one fractional damping acting on a part of the boundary of the domain. Next, we study the stability of the system of coupled multi-dimensional wave equation with one fractional damping acting on a part of the boundary of the domain. Finally, we study the stability of the multidimensional waves equations with locally viscoelastic damping of Kelvin-Voight is applied for one equation around the boundary of the domain
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Lytle, Joshua W. "Stability for Traveling Waves." BYU ScholarsArchive, 2011. https://scholarsarchive.byu.edu/etd/3063.
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In this work we present some of the general theory of shock waves and their stability properties. We examine the concepts of nonlinear stability and spectral stability, noting that for certain classes of equations the study of nonlinear stability is reduced to the analysis of the spectra of the linearized eigenvalue problem. A useful tool in the study of spectral stability is the Evans function, an analytic function whose zeros correspond to the eigenvalues of the linearized eigenvalue problem. We discuss techniques for numerical Evans function computation that ensure analyticity, allowing standard winding number arguments and rootfinding methods to be used to locate eigenvalues. The Evans function is then used to study the spectra of the high Lewis number combustion system, tracking eigenvalues in the right-half plane.
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Nakano, Yushi. "Stochastic Stability of Partially Expanding Maps via Spectral Approaches." Kyoto University, 2015. http://hdl.handle.net/2433/200463.
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Kyoto University (京都大学)0048
新制・課程博士
博士(人間・環境学)
甲第19200号
人博第741号
新制||人||178(附属図書館)
27||人博||741(吉田南総合図書館)
32192
京都大学大学院人間・環境学研究科共生人間学専攻
(主査)教授 宇敷 重廣, 教授 森本 芳則, 准教授 木坂 正史
学位規則第4条第1項該当
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Hendricks, Jeffrey James. "Spectral Stability of Weak Detonations in the Majda Model." BYU ScholarsArchive, 2013. https://scholarsarchive.byu.edu/etd/3626.
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Using analytical and numerical Evans-function techniques, we examine the spectral stability of weak-detonation-wave solutions of Majda's scalar model for a reacting gas mixture. We provide a proof of monotonicity of solutions. Using monotonicity we obtain a bound on possible unstable eigenvalues for weak-detonation-wave solutions that improves on the more general bound given by Humpherys, Lyng, and Zumbrun. We use a numerical approximation of the Evans function to search for possible unstable eigenvalues in the bounded region obtained by the energy estimate. For the parameter values tested, our results combined with the result of Lyng, Raoofi, Texier, and Zumbrun demonstrate that these waves are nonlinearly phase-asymptotically orbitally stable throughout the parameter space for which solutions were obtainable.
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Orynbayev, Murat. "Spectral response characterization and stability of polymer-fullerene solar cells /." [Beersheba] : Ben-Gurion University of the Negev, 2006. http://aranne5.lib.ad.bgu.ac.il/others/OrynbayevMurat.pdf.
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Höwing, Johannes [Verfasser]. "Spectral Stability of Solitary Waves and Undercompressive Shocks / Johannes Höwing." Konstanz : Bibliothek der Universität Konstanz, 2013. http://d-nb.info/1037917707/34.
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Chiquete, Carlos. "Stability and Receptivity of Idealized Detonations." Diss., The University of Arizona, 2011. http://hdl.handle.net/10150/145419.
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The linear receptivity and stability of plane idealized detonation with one-step Arrhenius type reaction kinetics is explored in the case of three-dimensional perturbations to a Zel'dovich-von Neumann-Doering base flow. This is explored in both overdriven and explicitly Chapman-Jouguet detonation. Additionally, the use of a multi-domain spectral collocation method for solving the conventional stability problem is explored within the context of normal-mode detonation. An extension of the stability analysis to confined detonations in a slightly porous walled tube is also carried out. Finally, an asymptotic analysis of a detonation with two-step reaction kinetics in the limit of large activation energy and for general overdrive and reaction order is performed yielding a nonlinear evolution equation for perturbations that produce stable limit cycle solutions.
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Corbett, Jennifer Leontine. "Sodium-cluster complexes : probability of formation, stability and mass spectra." Thesis, University of British Columbia, 2012. http://hdl.handle.net/2429/41966.
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Accurate knowledge of aerosol size and composition is important for the understanding of their effects on both the atmosphere and the health of those who inhale them. Ultrafine aerosols (0.25 nm to 10 nm radius) and small clusters (2 molecules to 0.25 nm radius) can only be detected down to radii of a few nanometers and no chemical information is provided. A photoion spectrometer was previously built and is used to dope clusters and ultrafine aerosols with Na atoms. The subsequent Na-cluster complexes are then ionized by a UV laser. This technique is currently believed to be the softest ionization method available. Calculations were done to predict the efficiency of this system because a direct comparison with other systems cannot be made over the complete size range. These calculations include, the probability of a particle colliding with a Na atom in the oven, the average lifetime of Na-cluster complexes and their photoionization efficiency. Calculations in this thesis focus on the collision probability and include predictions as to the most efficient temperature of the Na oven for Na pick up. The temperature of the Na oven was also found to experimentally agree with the calculations, because the vapor pressure of Na plays a significant role in the collision probability. Characterization of the sodium doping technique was done through experiments comparing direct XUV ionization with UV ionization of small Na doped clusters. The fragmentation of acetic acid clusters was characterized for the monomer and oligomers up to the tetramer. It was found that up to two acetic acid molecules evaporated from the clusters. The Na doping technique also reveals a significant difference in the size distribution as seen by UV and XUV laser ionization for large dimethyl ether and acetic acid clusters, and a smaller yet visible difference for large water clusters. Different UV wavelengths were also investigated including wavelengths above and below the ionization energy of a lone Na atom. The Na doping technique was found to be a softer technique than direct ionization with XUV light, because no fragmentation of molecules occurred and evaporation was minimized.
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Yamazoe, Shotaro. "Bifurcations and Spectral Stability of Solitary Waves in Nonlinear Wave Equations." Kyoto University, 2020. http://hdl.handle.net/2433/259759.
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Feleqi, Ermal. "Spectral stability estimates for the eigenfunctions of second order elliptic operators." Doctoral thesis, Università degli studi di Padova, 2009. http://hdl.handle.net/11577/3421876.
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The thesis deals with a new direction in the spectral stability problem of elliptic operators, namely to find estimates for the variation of eigenfunctions due to domain perturbation. Unlike the problem of the variation of the eigenvalues, this problem had practically never been investigated previously.
The approach developed in the thesis is based on the concept of "gap'' between linear operators. This concept, which measures the proximity of two unbounded linear operators, extends to the case of operators defined on different domains. Thereafter, there are obtained estimates of the gap between elliptic partial differential operators with homogeneous Dirichlet boundary conditions defined on different domains.
This then allows one to get estimates for the norm of the differences of eigenprojectors and, as a consequence, the desired estimates for the differences of appropriate eigenfunctions of operators defined on different domains, estimates expressed through the geometrical features, namely through the power of the Hausdorff-Pompeiu distance between the boundaries of the domains. The results obtained are new and very interesting and give a good contribution to the problem of spectral stability for differential operatorsLa tesi affronta una nuova direzione nel problema della stabilità spettrale degli operatori ellittici, precisamente di trovare stime per la variazione delle autofunzioni in seguito alla perturbazione del dominio. A differenza del problema della variazione degli autovalori, questo problema non era praticamente mai stato investigato in precedenza. L'approccio che è stato sviluppato nella tesi si basa sulla nozione di "gap'' tra operatori lineari. Questa nozione, che misura la vicinanza di due operatori lineari illimitati, si estende al caso di operatori definiti su domimi diversi. In seguito si ottengono delle stime per il gap tra operatori differenziali ellittici alle derivate parziali con condizioni al contorno di Dirichlet omogenee definiti su domini diversi. Ciò quindi permette di ottenere delle stime per la norma delle differenze degli autoproiettori e come conseguenza le stime volute per la norma delle differenze di appropriate autofunzioni di operatori definiti su domini diversi, stime espresse mediante caratteristiche geometiche dei domini, precisamente mediante potenza della distanza di Hausdorff-Pompeiu tra i bordi dei domini. I risultati ottenuti sono nuovi e molto interessanti e danno un buon contributo al problema della stabilità spettrale per operatori differenziali.
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Bassam, Maya. "Étude de la stabilité de quelques systèmes d'équations des ondes couplées sur des domaines bornés et non bornés." Thesis, Valenciennes, 2014. http://www.theses.fr/2014VALE0034/document.
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La thèse est portée essentiellement sur la stabilisation indirecte d’un système de deux équations des ondes couplées et sur la stabilisation frontière de poutre de Rayleigh.Dans le cas de la stabilisation d’un système d’équations d’onde couplées, le contrôle est introduit dans le système directement sur le bord du domaine d’une seule équation dans le cas d’un domaine borne ou à l’intérieur d’une seule équation mais dans le cas d’un domaine non borné. La nature du système ainsi couplé dépend du couplage des équations et de la nature arithmétique des vitesses de propagations, et ceci donne divers résultats pour la stabilisation polynomiale ainsi la non stabilité.Dans le cas de la stabilisation de poutre de Rayleigh, l’équation est considérée avec un seul contrôle force agissant sur bord du domaine. D’abord, moyennant le développement asymptotique des valeurs propres et des vecteurs propres du système non contrôlé, un résultat d’observabilité ainsi qu’un résultat de bornétude de la fonction de transfert correspondant sont obtenus. Alors, un taux de décroissance polynomial de l’énergie du système est établi. Ensuite, moyennant une étude spectrale combinée avec une méthode fréquentielle, l’optimalité du taux obtenu est assuréeThe thesis is driven mainly on indirect stabilization system of two coupled wave equations and the boundary stabilization of Rayleigh beam equation. In the case of stabilization of a coupled wave equations, the Control is introduced into the system directly on the edge of the field of a single equation in the case of a bounded domain or inside a single equation but in the case of an unbounded domain. The nature of thus coupled system depends on the coupling equations and arithmetic Nature of speeds of propagation, and this gives different results for the polynomial stability and the instability. In the case of stabilization of Rayleigh beam equation, we consider an equation with one control force acting on the edge of the area. First, using the asymptotic expansion of the eigenvalues and vectors of the uncontrolled system an observability result and a result of boundedness of the transfer function are obtained. Then a polynomial decay rate of the energy of the system is established. Then through a spectral study combined with a frequency method, optimality of the rate obtained is assured
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Toumazou, Vincent. "Portraits spectraux de matrices : un outil d'analyse de la stabilité." Nancy 1, 1996. http://www.theses.fr/1996NAN10273.
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La détermination des valeurs propres et/ou des vecteurs propres de matrices de grande taille occupe une place importante dans le calcul scientifique. Cependant, le traitement numérique de problèmes physiques couples génère souvent des problèmes de valeurs propres de grande taille non-normaux qui peuvent alors exhiber de l'instabilité spectrale. Il est crucial pour le scientifique de disposer d'outils capables de détecter et/ou de mesurer l'instabilité spectrale des matrices qu'il utilise et de prédire et/ou quantifier les perturbations résultant sur la solution calculée a précision finie. Ce document passe en revue les outils théoriques classiques utilises dans l'estimation de l'erreur et montre leurs limites en présence d'instabilité spectrale forte. La non-normalité et ses liens avec l'instabilité spectrale sont particulièrement détaillés: la taxonomie établie permet de clarifier les notions de matrices normales et fortement non-normales. Nous montrons comment les quantificateurs classiques de la non-normalité ne parviennent pas à prédire de manière satisfaisante l'instabilité spectrale. Une alternative basée sur les pseudospectres est alors proposée dans le cas des matrices et des faisceaux de matrices. Nous étudions la représentation des pseudospectres par des portraits spectraux ainsi que les différentes interprétations qui peuvent en être faites à partir de nombreux exemples notamment issus de la physique. On peut ainsi comprendre comment les portraits spectraux permettent de visualiser l'instabilité spectrale qui découle de la non-normalité. Enfin, différentes méthodes numériques pour le calcul des portraits spectraux sont exposées. L’implantation informatique de la méthode retenue, celle de l'équation normale, est détaillée dans le cadre d'une architecture parallèle à mémoire distribuée, rendant cet outil utilisable dans un cadre industriel.
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Reizer, Gabriella v. "Stability Selection of the Number of Clusters." Digital Archive @ GSU, 2011. http://digitalarchive.gsu.edu/math_theses/98.
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Selecting the number of clusters is one of the greatest challenges in clustering analysis. In this thesis, we propose a variety of stability selection criteria based on cross validation for determining the number of clusters. Clustering stability measures the agreement of clusterings obtained by applying the same clustering algorithm on multiple independent and identically distributed samples. We propose to measure the clustering stability by the correlation between two clustering functions. These criteria are motivated by the concept of clustering instability proposed by Wang (2010), which is based on a form of clustering distance. In addition, the effectiveness and robustness of the proposed methods are numerically demonstrated on a variety of simulated and real world samples.
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Issa, Ibtissam. "Some results on the stabilization of elastic/viscoelastic transmission problems with Kelvin-Voigt or fractional Kelvin-Voigt damping." Thesis, Aix-Marseille, 2021. http://theses.univ-amu.fr.lama.univ-amu.fr/211207_ISSA_690cu840ucxbzr880kpmyt859oe_TH.pdf.
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Cette thèse est consacrée à l'étude de la stabilisation de certains systèmes localement couplés. Tout d'abord, nous étudions la stabilité d'équations d'onde couplées unidimensionnelles avec deux amortissements visqueux intérieurs non lisses où nous établissons une stabilité exponentielle. Dans un second temps, nous étudions la stabilisation d'équations d'onde localement couplées avec un seul amortissement viscoélastique interne de type Kelvin-Voigt. L'amortissement et les coefficients de couplage ne sont pas lisses. En utilisant une approche spectrale, nous démontrons la stabilité non uniforme du système. Ensuite, en utilisant une approche dans le domaine fréquentiel, combinée à une technique de multiplicateur par morceaux et à la construction d'un nouveau multiplicateur satisfaisant quelques équations différentielles ordinaires, nous montrons que l'énergie de la solution lisse du système décroît polynomiale. Troisièmement, nous étudions la décroissance énergétique de systèmes hyperboliques de type onde-onde, onde-Euler Bernoulli et faisceau-faisceau. En effet, les deux équations sont couplées par liaison limite avec un seul amortissement fractionnaire Kelvin Voigt localisé non lisse. Nous établissons un taux de décroissance d'énergie polynomial. Enfin, nous étudions la stabilité d'un système multidimensionnel de deux équations d'onde couplées par des vitesses avec un seul amortissement Kelvin-Voigt localisé non lisse. En utilisant une analyse spectrale, nous prouvons la stabilité non uniforme du système. Nous établissons des résultats de stabilité polynomiale en considérant différentes conditions géométriques sur les domaines de couplage et d'amortissementThis thesis is devoted to study the stabilization of some locally coupled systems. First, we study the stability of a one-dimensional coupled wave equations with two interior non smooth viscous dampings where we establish exponential stability. Second, we study the stabilization of a locally coupled wave equations with only one internal viscoelastic damping of Kelvin-Voigt type. Both the damping and the coupling coefficients are non smooth. Using a spectrum approach, we prove the non-uniform stability of the system. Next, using a frequency domain approach, combined with a piecewise multiplier technique and the construction of a new multiplier satisfying some ordinary differential equations, we show that the energy of the smooth solution of the system decays polynomially. Third, we investigate the energy decay of hyperbolic systems of wave-wave, wave-Euler Bernoulli beam and beam-beam types. Indeed, the two equations are coupled through boundary connection with only one localized non smooth fractional Kelvin Voigt damping. We establish a polynomial energy decay rate. Finally, we study the stability of a multidimensional system of two wave equations coupled by velocities with only one localized non-smooth Kelvin-Voigt damping. By using a spectral analysis, we prove the non uniform stability of the system. Further, using a frequency domain approach combined with a multiplier technique, we establish some polynomial stability results by considering different geometric conditions on the coupling and the damping domains. In addition, in the absence of any geometric condition, we establish two polynomial energy decay rates of the system on a square domain
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Lerbinger, Klaus. "Problèmes spectraux en MHD : effets non idéaux, stabilité du kink interne." Paris 11, 1988. http://www.theses.fr/1988PA112109.
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La magnétohydrodynamique présente un grand intérêt pour l'étude des plasmas de fusion magnétiquement confinés. Les équations linéarisées de la MHD idéale peuvent être mises sous une forme adaptée à l'analyse spectrale; le système est alors décrit par des opérateurs linéaires auto-adjoints. La prise en compte d'une résistivité finie affecte profondément le système. Les opérateurs linéaires ne sont plus auto-adjoints; de ce fait, on ne peut plus recourir à un principe variationnel. Les valeurs propres deviennent complexes et de nouvelles instabilités apparaissent. Nous présentons un code numérique qui résout les équations linéarisées de la MHD résistive en géométrie cylindrique; un choix judicieux des éléments finis (des polynômes cubiques d'Hermite et de polynômes quadratiques} s'avère nécessaire pour approximer correctement la totalité du spectre. Le problème aux valeurs propres généralisé est résolu par l'itération vectorielle inverse qui se montre la plus efficace. Nous discutons également un code d'évolution temporelle implicite fondé sur la même discrétisation. Les modes d'Alfvèn forment une composante continue du spectre. Dans le cas résistif ces modes singuliers disparaissent; les fonctions propres deviennent régulières et les valeurs propres se placent sur des courbes bien définies du plan complexe. Nous étudions également le spectre d'une couche de plasma dans laquelle la conductivité thermique est radiative. Nous montrons qu'en géométrie torique le kink interne est toujours instable pour des profils de courant creux et le taux de croissance est plus élevé que celui du kink interne pour un profil de courant monotone. Des simulations non linéaires de cette instabilité en géométrie cylindrique montrent qu'elle s'achève par une disruption ou une saturation, suivant l'énergie disponible.
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Xiao, Lan. "Etude physico-chimique de la complexation de divers ions metalliques (cu**(2+), ni**(2+) et co**(2+)) avec des dipeptides l-l contenant des chaines laterales non-coordinantes." Paris 7, 1988. http://www.theses.fr/1988PA077170.
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Stabilite des complexes en fonction de l'interaction des chaines laterales du dipeptide et le solvant. Mesures potentiometriques calorimetriques, caracterisation spectrometrique et structure moleculaire
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Khan, Mustayeen Ahmed. "Stabilité, spectres électroniques et structures des halogénocuprates en milieu protique." Grenoble 2 : ANRT, 1988. http://catalogue.bnf.fr/ark:/12148/cb37614662r.
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Wächtler, Johannes [Verfasser]. "Spectral stability of small-amplitude traveling waves via geometric singular perturbation theory / Johannes Wächtler." Konstanz : Bibliothek der Universität Konstanz, 2012. http://d-nb.info/1024853799/34.
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Arnqvist, Johan. "Mean Wind and Turbulence Conditions in the Boundary Layer above Forests." Doctoral thesis, Uppsala universitet, Luft-, vatten och landskapslära, 2015. http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-237764.
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As wind turbines have grown, new installation areas become possible. Placing wind turbines in forested landscapes introduce uncertainties to the wind resource estimation. Even though close-to-canopy processes have been studied intensively during the last thirty years, the focus has mostly been on exchange processes and the height span of the studies has been below the rotor of a modern wind turbine. This thesis contains analysis of new measurements from a 138 m high tower in a forested landscape. The previous knowledge of near-canopy processes is extended to the region above the roughness sublayer. It is shown that above the roughness sublayer, the surface layer behaves as over low vegetation, and Monin-Obukhov similarity is shown to hold for several variables. However, in stable stratification, effects that could be linked to the boundary layer depth are shown to be present in the measurements. These include wind turning with height, the behaviour of the turbulence length scale and the curvature of the wind profile. Two new analytical models are presented in the thesis. One is a flux-profile expression in the roughness sublayer, which allows for analytical integration of the wind gradient. The model suggests that the roughness-sublayer effect depends on stratification and that the aerodynamic roughness length changes with stability. A decrease of roughness length in stable stratification is confirmed with a new method to determine the roughness length using measurements from the 138 m tower. The other model determines the spectral tensor in stable stratification using analytical solution to the rapid distortion equations for stratified shear flow, with homogeneous stratification and shear. By using a formulation for the integration time of the distortions of an isotropic spectrum, a model is derived which provides the cross spectra of velocity and temperature at any two given points in space. Finally the existence of waves in the wind over forests is investigated and it is concluded that the Kelvin-Helmholtz instability can create waves which are coherent in time and exist over the entire height span of wind turbine rotors. Linear wave theory is shown to be able to explain certain features of the waves.Vindforsk III, Wind power in forests
Vindforsk IV, Forest wind
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Řehák, Petr. "Prvoprincipiální analýza stability krystalů pevných látek." Doctoral thesis, Vysoké učení technické v Brně. Fakulta strojního inženýrství, 2013. http://www.nusl.cz/ntk/nusl-234177.
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The aim of the author’s research in the period of his PhD study was the analysis of mechanical stability of cubic crystals under external loading. This work demonstrates several methods used for a study of mechanical stability of fcc crystals (C, Al, Ir, Pt, Au) during isotropic (hydrostatic) tensile loading. Ab initio methods were used for this purpose. Studied crystals were subjected to simulated isotropic tensile deformation and an analysis of elastic stability was performed. This analysis shows that first elastic instability in Al, Pt and Au crystals corresponds to vanishing of the trigonal shear modulus and diamond, Ir remains stable up to the state of maximum isotropic stress. According to the calculated band structure diamond crystal preserves his insulating character up to the onset of instability. Consequently, phonon spectra of all crystals were calculated using the linear response method and their dynamic stability was assessed. Obtained results reveal soft phonon modes in Al, Pt and Ir before an occurrence of elastic instability. Selected short-wavelength instabilities are confirmed using models of microscopic deformation and also using dispersion curves obtained by a supercell method. The observed instabilities lower critical strains related to the volumetric instability up to 40 % whereas the reduction of critical stress is by 20 % at the most.
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Venzl, Hannah [Verfasser], and Andreas [Akademischer Betreuer] Buchleitner. "Ultracold bosons in tilted optical lattices – impact of spectral statistics on simulability, stability, and dynamics = Ultrakalte Bosonen in optischen Gittern - der Einfluss spektraler Statistik auf Simulierbarkeit, Stabilität und Dynamik." Freiburg : Universität, 2011. http://d-nb.info/1123459029/34.
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Liefvendahl, Mattias. "Stability results for viscous shock waves and plane Couette flow." Doctoral thesis, Stockholm, 2001. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-3241.
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Klaiber, Andreas [Verfasser]. "On the Spectral Stability of Internal Solitary Waves in Fluids with Density Stratification / Andreas Klaiber." Konstanz : Bibliothek der Universität Konstanz, 2013. http://d-nb.info/1045840556/34.
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Lerbinger, Klaus. "Problèmes spectraux en MHD effets non idéaux, stabilité du Kink interne /." Grenoble 2 : ANRT, 1988. http://catalogue.bnf.fr/ark:/12148/cb376152381.
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OGNIBENE, ROBERTO. "Monotonicity formulas and blow-up methods for the study of spectral stability and fractional obstacle problems." Doctoral thesis, Università degli studi di Pavia, 2021. http://hdl.handle.net/11571/1431735.
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The present dissertation is essentially divided into two parts.
In the first part, we investigate questions of spectral stability for eigenvalue problems driven by the Laplace operator, under certain specific kinds of singular perturbations. More precisely, we start by considering the spectrum of the Laplacian on a fixed, bounded domain with prescribed homogeneous boundary conditions (of pure Dirichlet or Neumann type); then, we introduce a singular perturbation of the problem, which gives rise to a perturbed sequence of eigenvalues. Our goal is to understand the asymptotic behavior of the perturbed spectrum as long as the perturbation tends to disappear. In particular, we consider two different types of singular perturbation. On one hand, in the case of homogeneous Dirichlet boundary conditions, we consider a perturbation of the domain, which consists in attaching a thin cylindrical tube to the fixed limit domain and let its section shrink to a point. In this framework, we combine energy estimates coming from a tailor-made Almgren type monotonicity formula with the Courant-Fischer min-max characterization and then we perform a careful blow-up analysis for scaled eigenfunctions; with these ingredients, we identify the sharp rate of convergence of a perturbed eigenvalue in the case in which it is approaching a simple eigenvalue of the limit problem. On the other hand, we deal with a perturbation of the boundary conditions. More specifically, we start with the homogeneous Neumann eigenvalue problem for the Laplacian and we perturb it by prescribing zero Dirichlet boundary conditions on a small subset of the boundary. In this context, we describe the sharp asymptotic behavior of a perturbed eigenvalue when it is converging to a simple eigenvalue of the limit Neumann problem. In particular, the first term in the asymptotic expansion turns out to depend on the Sobolev capacity of the subset where the perturbed eigenfunction is vanishing. We also provide a more ‘explicit’ expression for the eigenvalue variation in the particular case of Dirichlet boundary conditions imposed on a subset which is scaling to a point.
In the second part of this thesis, we deal with two problems, both governed by the fractional Laplace operator, i.e. the power of order between 0 and 1 of the classical (negative) Laplacian. First, we address the question of positivity of a nonlocal Schrödinger operator, driven by the fractional Laplacian and with singular multipolar Hardy-type potentials. Namely, we provide necessary and sufficient conditions on the coefficients of the potential for the existence of a configuration of poles that ensures the positivity of the corresponding Schrödinger operator. This result is based, in turn, on a criterion in the spirit of the Agmon-Allegretto-Piepenbrink principle and on a tool fitting in the theory of localization of binding. The second topic we investigate in this part concerns geometric properties of the free boundary of solutions of a two-phase penalized obstacle-type problem for the fractional Laplacian. In view of the Caffarelli-Silvestre extension, we can interpret it as a thin obstacle-type problem driven by a second-order differential operator living in one dimension more and with a Muckenhoupt weight, that can be either singular or degenerate on the thin space. Working in this framework, by means of Almgren and Monneau type monotonicity formulas and blow-up analysis, we first prove a classification of the possible vanishing orders on the thin space and, as a consequence, the boundary strong unique continuation principle. We finally establish a stratification result for the nodal set (which coincides with the free boundary) on the thin space and we provide sharp estimates on the Hausdorff dimension of its regular and singular part.
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Hagen, Thomas Ch. "Elongational Flows in Polymer Processing." Diss., Virginia Tech, 1998. http://hdl.handle.net/10919/29437.
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The production of long, thin polymeric fibers is a main objective of the textile industry. Melt-spinning is a particularly simple and effective technique. In this work, we shall discuss the equations of melt-spinning in viscous and viscoelastic flow. These quasilinear hyperbolic equations model the uniaxial extension of a fluid thread before its solidification.
We will address the following topics: first we shall prove existence, uniqueness, and regularity of solutions. Our solution strategy will be developed in detail for the viscous case. For non-Newtonian and isothermal flows, we shall outline the general ideas. Our solution technique consists of energy estimates and fixed-point arguments in appropriate Banach spaces. The existence result for a simple transport equation is the key to understanding the quasilinear case. The second issue of this exposition will be the stability of the unforced frost line formation. We will give a rigorous justification that, in the viscous regime, the linearized equations obey the ``Principle of Linear Stability''. As a consequence, we are allowed to relate the stability of the associated strongly continuous semigroup to the numerical resolution of the spectrum of its generator. By using a spectral collocation method, we shall derive numerical results on the eigenvalue distribution, thereby confirming prior results on the stability of the steady-state solution.Ph. D.
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Portmann, Fabian. "Spectral Inequalities and Their Applications in Quantum Mechanics." Doctoral thesis, KTH, Matematik (Avd.), 2014. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-145210.
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The work presented in this thesis revolves around spectral inequalities and their applications in quantum mechanics. In Paper A, the ground state energy of an atom confined to two dimensions is analyzed in the limit when the charge of the nucleus Z becomes very large. The main result is a two-term asymptotic expansion of the ground state energy in terms of Z. Paper B deals with Hardy inequalities for the kinetic energy of a particle in the presence of an external magnetic field. If the magnetic field has a non-trivial radial component, we show that Hardy’s classical lower bound can be improved by an extra term depending on the magnetic field. In Paper C we study interacting Bose gases and prove Lieb-Thirring type estimates for several types of interaction potentials, such as the hard-sphere interaction in three dimensions, the hard-disk interaction in two dimensions as well as homogeneous potentials.QC 20140520
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Ford, Wesley. "The Advancement of Stable, Efficient and Parallel Acceleration Methods for the Neutron Transport Equation." Thesis, Université Paris-Saclay (ComUE), 2019. http://www.theses.fr/2019SACLX105/document.
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Dans cet article, nous proposons une nouvelle bibliothèque de techniques non linéaires pour accélérer l’équation de transport en ordonnées discrètes. Deux nouveaux types de méthodes d'accélération non linéaire appelées méthode de rééquilibrage spatialement variable (SVRM) et accélération de matrice de réponse (RMA), respectivement, sont proposées et étudiées. La première méthode, SVRM, est basée sur le calcul de la variation spatiale de premier ordre de l'équation de la balance des neutrons. RMA, est une méthode DP0 qui utilise la connaissance de l'opérateur de transport pour former une relation cohérente. Deux variantes distinctes de RMA, appelées respectivement Explicit-RMA (E-RMA) et Balance (B-RMA), sont dérivées. Les propriétés de convergence des deux méthodes d'accélération sont étudiées pour deux schémas d'itération différents de l'opérateur de transport de la méthode des caractéristiques (MOC) pour une dalle 1D, en utilisant une analyse spectrale et une analyse de Fourier. Sur la base des résultats de la comparaison 1D, seuls les outils RMA et CMFD ont été implémentés dans la bibliothèque. Les performances de RMA sont comparées à celles de CMFD en utilisant les tests 3D C5G7, ZPPR et UH12. Les schémas de résolution parallèles et séquentiels sont considérés. L'analyse des résultats indique que les deux variantes de RMA ont une efficacité et une stabilité améliorées par rapport au CMFD, pour les matériaux à diffusion optique. De plus, le RMA montre une amélioration importante de la stabilité et de l'efficacité lorsque la géométrie est décomposée spatialement. Pour obtenir des performances numériques optimales, une combinaison de RMA et de CMFD est suggérée. Une enquête plus approfondie sur l'utilisation et l'amélioration de la RMA est proposée. De plus, de nombreuses idées pour étendre les fonctionnalités de la bibliothèque sont présentéesIn this paper we propose a new library of non-linear techniques for accelerating the discrete-ordinates transport equation. Two new types of nonlinear acceleration methods called Spatially Variant Rebalancing Method (SVRM) and Response Matrix Acceleration (RMA), respectively, are proposed and investigated. The first method, SVRM, is based on the computation of the zeroth and first order spatial variation of the neutron balance equation. RMA, is a DP0 method that uses knowledge of the transport operator to form a consistent relationship. Two distinct variants of RMA, called Explicit-RMA (E-RMA) and Balance (B-RMA), respectively, are derived. The convergence properties of both acceleration methods are investigated for two different iteration schemes of the method of characteristics (MOC) transport operator for a 1D slab, using spectral and Fourier analysis. Based off the results of the 1D comparison, only RMA and CMFD were implemented in the library. The performance of RMA is compared to CMFD using the C5G7, ZPPR, and UH12 3D benchmarks. Both parallel and sequential solving schemes are considered. Analysis of the results indicates that both variants of RMA have improved effectiveness and stability relative to CMFD, for optically diffusive materials. Moreover, RMA shows great improvement in stability and effectiveness when the geometry is spatially decomposed. To achieve optimal numerical performance, a combination of RMA and CMFD is suggested. Further investigation into the use and improvement of RMA is proposed. As well, many ideas for extending the features of the library are presented
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Řehák, Petr. "Stabilita krystalů pevných látek z prvních principů." Master's thesis, Vysoké učení technické v Brně. Fakulta strojního inženýrství, 2009. http://www.nusl.cz/ntk/nusl-228571.
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This work deals with study of stability of solid crystals under isotropic loading. Ab initio methods were used for this purpose. Crystals of four fcc metals (Al, Cu, Ir, Au) and diamond were subjected to simulated isotropic tensile deformation and maximum value of isotropic stress was evaluated for them. Consequently, phonon spectra were calculated for several strain values in order to assess crystal stability. Phonon instabilities in dispersion curves of diamond, Al, Ir and Au appeared at strains lower than those corresponding to their decohesion. This appearance of instability determinates the value of ideal strength. However, significant reduction (by about 20%) was found only in the cases of Au and Ir.
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Hao, Zhaopeng. "High-order numerical methods for integral fractional Laplacian: algorithm and analysis." Digital WPI, 2020. https://digitalcommons.wpi.edu/etd-dissertations/612.
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The fractional Laplacian is a promising mathematical tool due to its ability to capture the anomalous diffusion and model the complex physical phenomenon with long-range interaction, such as fractional quantum mechanics, image processing, jump process, etc. One of the important applications of fractional Laplacian is a turbulence intermittency model of fractional Navier-Stokes equation which is derived from Boltzmann's theory. However, the efficient computation of this model on bounded domains is challenging as highly accurate and efficient numerical methods are not yet available. The bottleneck for efficient computation lies in the low accuracy and high computational cost of discretizing the fractional Laplacian operator. Although many state-of-the-art numerical methods have been proposed and some progress has been made for the existing numerical methods to achieve quasi-optimal complexity, some issues are still fully unresolved: i) Due to nonlocal nature of the fractional Laplacian, the implementation of the algorithm is still complicated and the computational cost for preparation of algorithms is still high, e.g., as pointed out by Acosta et al \cite{AcostaBB17} 'Over 99\% of the CPU time is devoted to assembly routine' for finite element method; ii) Due to the intrinsic singularity of the fractional Laplacian, the convergence orders in the literature are still unsatisfactory for many applications including turbulence intermittency simulations. To reduce the complexity and computational cost, we consider two numerical methods, finite difference and spectral method with quasi-linear complexity, which are summarized as follows. We develop spectral Galerkin methods to accurately solve the fractional advection-diffusion-reaction equations and apply the method to fractional Navier-Stokes equations. In spectral methods on a ball, the evaluation of fractional Laplacian operator can be straightforward thanks to the pseudo-eigen relation. For general smooth computational domains, we propose the use of spectral methods enriched by singular functions which characterize the inherent boundary singularity of the fractional Laplacian. We develop a simple and easy-to-implement fractional centered difference approximation to the fractional Laplacian on a uniform mesh using generating functions. The weights or coefficients of the fractional centered formula can be readily computed using the fast Fourier transform. Together with singularity subtraction, we propose high-order finite difference methods without any graded mesh. With the use of the presented results, it may be possible to solve fractional Navier-Stokes equations, fractional quantum Schrodinger equations, and stochastic fractional equations with high accuracy. All numerical simulations will be accompanied by stability and convergence analysis.