Dissertations / Theses on the topic 'Quantum theory – Mathematics; Group theory'
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Gupta, Neha. "Homotopy quantum field theory and quantum groups." Thesis, University of Warwick, 2011. http://wrap.warwick.ac.uk/38110/.
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The thesis is divided into two parts one for dimension 2 and the other for dimension 3. Part one (Chapter 3) of the thesis generalises the definition of an n-dimensional HQFT in terms of a monoidal functor from a rigid symmetric monoidal category X-Cobn to any monoidal category A. In particular, 2-dimensional HQFTs with target K(G,1) taking values in A are generated from any Turaev G-crossed system in A and vice versa. This is the generalisation of the theory given by Turaev into a purely categorical set-up. Part two (Chapter 4) of the thesis generalises the concept of a group-coalgebra, Hopf group-coalgebra, crossed Hopf group-coalgebra and quasitriangular Hopf group-coalgebra in the case of a group scheme. Quantum double of a crossed Hopf group-scheme coalgebra is constructed in the affine case and conjectured for the more general non-affine case. We can construct 3-dimensional HQFTs from modular crossed G-categories. The category of representations of a quantum double of a crossed Hopf group-coalgebra is a ribbon (quasitriangular) crossed group-category, and hence can generate 3-dimensional HQFTs under certain conditions if the category becomes modular. However, the problem of systematic finding of modular crossed G-categories is largely open.
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Mantke, Wolfgang Johann. "Picture independent quantum action principle." Diss., Georgia Institute of Technology, 1992. http://hdl.handle.net/1853/29850.
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Carruth, Nathan Thomas. "Classical Foundations for a Quantum Theory of Time in a Two-Dimensional Spacetime." DigitalCommons@USU, 2010. https://digitalcommons.usu.edu/etd/708.
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We consider the set of all spacelike embeddings of the circle S1 into a spacetime R1 × S1 with a metric globally conformal to the Minkowski metric. We identify this set and the group of conformal isometries of this spacetime as quotients of semidirect products involving diffeomorphism groups and give a transitive action of the conformal group on the set of spacelike embeddings. We provide results showing that the group of conformal isometries is a topological group and that its action on the set of spacelike embeddings is continuous. Finally, we point out some directions for future research.
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Gajewski, David C. "Analysis of Groups Generated by Quantum Gates." Connect to full text in OhioLINK ETD Center, 2009. http://rave.ohiolink.edu/etdc/view?acc_num=toledo1250224470.
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Cooney, Nicholas. "Quantum multiplicative hypertoric varieties and localization." Thesis, University of Oxford, 2014. http://ora.ox.ac.uk/objects/uuid:17d0824f-e8f2-4cb7-9e84-dd3850a9e2a2.
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In this thesis, we consider q-deformations of multiplicative Hypertoric varieties, where q∈𝕂x for 𝕂 an algebraically closed field of characteristic 0. We construct an algebra Dq of q-difference operators as a Heisenberg double in a braided monoidal category. We then focus on the case where q is specialized to a root of unity. In this setting, we use Dq to construct an Azumaya algebra on an l-twist of the multiplicative Hypertoric variety, before showing that this algebra splits over the fibers of both the moment and resolution maps. Finally, we sketch a derived localization theorem for these Azumaya algebras.
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Laugwitz, Robert. "Braided Hopf algebras, double constructions, and applications." Thesis, University of Oxford, 2015. http://ora.ox.ac.uk/objects/uuid:ddcb459f-c3b4-40dd-9936-6bad6993ce8c.
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This thesis contains four related papers which study different aspects of double constructions for braided Hopf algebras. The main result is a categorical action of a braided version of the Drinfeld center on a Heisenberg analogue, called the Hopf center. Moreover, an application of this action to the representation theory of rational Cherednik algebras is considered. Chapter 1 : In this chapter, the Drinfeld center of a monoidal category is generalized to a class of mixed Drinfeld centers. This gives a unified picture for the Drinfeld center and a natural Heisenberg analogue. Further, there is an action of the former on the latter. This picture is translated to a description in terms of Yetter-Drinfeld and Hopf modules over quasi-bialgebras in a braided monoidal category. Via braided reconstruction theory, intrinsic definitions of braided Drinfeld and Heisenberg doubles are obtained, together with a generalization of the result of Lu (1994) that the Heisenberg double is a 2-cocycle twist of the Drinfeld double for general braided Hopf algebras. Chapter 2 : In this chapter, we present an approach to the definition of multiparameter quantum groups by studying Hopf algebras with triangular decomposition. Classifying all of these Hopf algebras which are of what we call weakly separable type, we obtain a class of pointed Hopf algebras which can be viewed as natural generalizations of multiparameter deformations of universal enveloping algebras of Lie algebras. These Hopf algebras are instances of a new version of braided Drinfeld doubles, which we call asymmetric braided Drinfeld doubles. This is a generalization of an earlier result by Benkart and Witherspoon (2004) who showed that two-parameter quantum groups are Drinfeld doubles. It is possible to recover a Lie algebra from these doubles in the case where the group is free and the parameters are generic. The Lie algebras arising are generated by Lie subalgebras isomorphic to sl2. Chapter 3 : The universal enveloping algebra U(trn) of a Lie algebra associated to the classical Yang-Baxter equation was introduced in 2006 by Bartholdi-Enriquez-Etingof-Rains where it was shown to be Koszul. This algebra appears as the An-1 case in a general class of braided Hopf algebras in work of Bazlov-Berenstein (2009) for any complex reection group. In this chapter, we show that the algebras corresponding to the series Bn and Dn, which are again universal enveloping algebras, are Koszul. This is done by constructing a PBW-basis for the quadratic dual. We further show how results of Bazlov-Berenstein can be used to produce pairs of adjoint functors between categories of rational Cherednik algebra representations of different rank and type for the classical series of Coxeter groups. Chapter 4 : Quantum groups can be understood as braided Drinfeld doubles over the group algebra of a lattice. The main objects of this chapter are certain braided Drinfeld doubles over the Drinfeld double of an irreducible complex reflection group. We argue that these algebras are analogues of the Drinfeld-Jimbo quantum enveloping algebras in a setting relevant for rational Cherednik algebra. This analogy manifests itself in terms of categorical actions, related to the general Drinfeld-Heisenberg double picture developed in Chapter 2, using embeddings of Bazlov and Berenstein (2009). In particular, this work provides a class of quasitriangular Hopf algebras associated to any complex reflection group which are in some cases finite-dimensional.
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Majard, Dany. "Cubical categories, TQFTs and possible new representations for the Poincare group." Diss., Kansas State University, 2012. http://hdl.handle.net/2097/14139.
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Doctor of PhilosophyDepartment of Mathematics
Louis Crane
In this thesis we explore the possibilities of obtaining Topological Quantum Field Theories using cobordisms with corners to break further down in the structure of manifolds of a given dimension. The algebraic data obtained is described in the language of higher category theory, more precisely in its cubical approach which we explore here as well. Interesting connections are proposed to some important objects in Physics: the representations of the Poincaré group. Finally we will describe in great details the topological tools needed to describe the categories of cobordisms with corners and give some conjectures on their nature.
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Boixeda, Alvarez Pablo. "Affine Springer fibers and the representation theory of small quantum groups and related algebras." Thesis, Massachusetts Institute of Technology, 2020. https://hdl.handle.net/1721.1/126920.
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Thesis: Ph. D., Massachusetts Institute of Technology, Department of Mathematics, May, 2020Cataloged from the official PDF of thesis.
Includes bibliographical references (pages 125-128).
This thesis deals with the connections of Geometry and Representation Theory. In particular we study the representation theory of small quantum groups and Frobenius kernels and the geometry of an equivalued affine Springer fiber Fl[subscript ts] for s a regular semisimple element. In Chapter 2 we relate the center of the small quantum group with the cohomology of the above affine Springer fiber. This includes joint work with Bezrukavnikov, Shan and Vaserot. In Chapter 3 we study the geometry of the affine Springer fiber and in particular understand the fixed points of a torus action contained in each component. In Chapter 4 we further have a collection of algebraic results on the representation theory of Frobenius kernels. In particular we state some results pointing towards some construction of certain partial Verma functors and we compute this in the case of SL₂. We also compute the center of Frobenius kernels in the case of SL₂ and state a conjecture on a possible inductive construction of the general center.
by Pablo Boixeda Alvarez.
Ph. D.
Ph.D. Massachusetts Institute of Technology, Department of Mathematics
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Ho, Yanfang. "Group theoretical analysis of in-shell interaction in atoms." Scholarly Commons, 1985. https://scholarlycommons.pacific.edu/uop_etds/487.
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A group theoretic approach to Layzer's 1/2 expansion method is explored. In part this builds on earlier work of Wulfman(2), of Moshinsky et al(l4), and of Sinanoglu, Herrick(lS), and Kellman (16) on second row atoms.
I investigate atoms with electrons in the 3s-3p-3d shell and find:
1. Wulfman's constant of motion accurately predicts configuration mixing for systems with two to eight electrons in the 3s-3p subshell.
2. The same constant of motion accurately predicts configuration mixing for systems with two electrons in the 3s-3p-3d shell.
3. It accurately predicts configuration mixing in systems of high angular momentum L and of high spin angular momentum S containing three electrons in the 3s-3p-3d shell, but gives less accurate results when L and S are both small.
I also show how effective nuclear charges may be calculated by a group theoretical approach. In addition I explore several new methods for expressing electron repulsion operators in terms of operators of the 80(4,2) dynamical group of one - electron atoms.
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Xu, Guang-Hui. "Exploratory studies of group theoretic methods in atomic physics." Scholarly Commons, 1989. https://scholarlycommons.pacific.edu/uop_etds/2189.
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The properties of a physical system are determined by its equation of motion, and every such equation admits one-parameter groups which keep the equation invariant. Thus, for a particular system, if one can find the generator of a one-parameter group which keeps the equation and some further function or functional invariant, then one can change this system into others by changing the parameter, while keeping some properties constant. In this way, one can tell why different systems have some common properties. More importantly, one can use this method to find relationships between the physical properties of different systems.
In the next section, we will illustrate the group theoretic approach by applying it to systems of two coupled oscillators and the hydrogen molecular ion. In section III of this thesis, we will investigate the helium atom system, considering both classical and quantum cases. In the quantum case our attention will be concentrated on the Schrodinger equation in matrix form. We will use a finite set of wavefunctions as our basis. Hence the results obtained will be approximate.
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Poulain, D'Andecy Loïc. "Algèbres de Hecke cyclotomiques : représentations, fusion et limite classique." Phd thesis, Aix-Marseille Université, 2012. http://tel.archives-ouvertes.fr/tel-00748920.
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Une approche inductive est développée pour la théorie des représentations de la chaîne des algèbres de Hecke cyclotomiques de type G(m,1,n). Cette approche repose sur l'étude du spectre d'une famille commutative maximale, formée par les analogues des éléments de Jucys-Murphy. Les représentations irréductibles, paramétrées par les multi-partitions, sont construites avec l'aide d'une nouvelle algèbre associative, dont l'espace vectoriel sous-jacent est le produit tensoriel de l'algèbre de Hecke cyclotomique avec l'algèbre associative libre engendrée par les multi-tableaux standards. L'analogue de cette approche est présentée pour la limite classique, c'est-à-dire la chaîne des groupes de réflexions complexes de type G(m,1,n). Dans une seconde partie, une base des algèbres de Hecke cyclotomiques est donnée et la platitude de la déformation est montrée sans utiliser la théorie des représentations. Ces résultats sont généralisés aux algèbres de Hecke affines de type A. Ensuite, une procédure de fusion est présentée pour les groupes de réflexions complexes et les algèbres de Hecke cyclotomiques de type G(m,1,n). Dans les deux cas, un ensemble complet d'idempotents primitifs orthogonaux est obtenu par évaluation consécutive d'une fonction rationnelle. Dans une troisième partie, une nouvelle présentation est obtenue pour les sous-groupes alternés de tous les groupes de Coxeter. Les générateurs sont reliés aux arêtes orientées du graphe de Coxeter. Cette présentation est ensuite étendue, pour tous les types, aux extensions spinorielles des groupes alternés, aux algèbres de Hecke alternées et aux sous-groupes alternés des groupes de tresses.
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McSorley, J. P. "Topics in group theory." Thesis, University of Oxford, 1986. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.376929.
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Abolfathe, Beikidezfuli Salman. "Quantum proof systems and entanglement theory." Thesis, Massachusetts Institute of Technology, 2009. http://hdl.handle.net/1721.1/50594.
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Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Mathematics, 2009.Includes bibliographical references (p. 99-106).
Quantum complexity theory is important from the point of view of not only theory of computation but also quantum information theory. In particular, quantum multi-prover interactive proof systems are defined based on complexity theory notions, while their characterization can be formulated using LOCC operations. On the other hand, the main resource in quantum information theory is entanglement, which can be considered as a monotonic decreasing quantity under LOCC maps. Indeed, any result in quantum proof systems can be translated to entanglement theory, and vice versa. In this thesis I mostly focus on quantum Merlin-Arthur games as a proof system in quantum complexity theory. I present a new complete problem for the complexity class QMA. I also show that computing both the Holevo capacity and the minimum output entropy of quantum channels are NP-hard. Then I move to the multiple-Merlin-Arthur games and show that assuming some additivity conjecture for entanglement of formation, we can amplify the gap in QMA(2) protocols. Based on the same assumption, I show that the QMA(k)-hierarchy collapses to QMA(2). I also prove that QMAlog(2), which is defined the same as QMA(2) except that the size of witnesses is logarithmic, with the gap n-(3+e) contains NP. Finally, motivated by the previous results, I show that the positive partial transpose test gives no bound on the trace distance of a given bipartite state from the set of separable states.
by Salman Abolfathe Beikidezfuli.
Ph.D.
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Evans, D. M. "Some topics in group theory." Thesis, University of Oxford, 1985. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.355748.
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Nicholson, Julia. "Otto Hölder and the development of group theory and Galois theory." Thesis, University of Oxford, 1993. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.333485.
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Grenham, Dermot. "Some topics in nilpotent group theory." Thesis, University of Oxford, 1988. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.329954.
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Girolami, Davide. "Quantum correlations in information theory." Thesis, University of Nottingham, 2013. http://eprints.nottingham.ac.uk/13397/.
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The project concerned the study of quantum correlations (QC) in compound systems, i.e. statistical correlations more general than entanglement which are predicted by quantum mechanics but not described in any classical scenario. I aimed to understand the technical and operational properties of the measures of QC, their interplay with entanglement quantifiers and the experimental accessibility. In the first part of my research path, after having acquired the conceptual and technical rudiments of the project, I provided solutions for some computational issues: I developed analytical and numerical algorithms for calculating bipartite QC in finite dimensional systems. Then, I tackled the problem of the experimental detection of QC. There is no Hermitian operator associated with entanglement measures, nor with QC ones. However, the information encoded in a density matrix is redundant to quantify them, thus the full knowledge of the state is not required to accomplish the task. I reported the first protocol to measure the QC of an unknown state by means of a limited number of measurements, without performing the tomography of the state. My proposal has been implemented experimentally in a NMR (Nuclear Magnetic Resonance) setting. In the final stage of the project, I explored the foundational and operational merits of QC. I showed that the QC shared by two subsystems yield a genuinely quantum kind of uncertainty on single local observables. The result is a promising evidence of the potential exploitability of separable (unentangled) states for quantum metrology in noisy conditions.
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Ali, David Benedict. "Aspects of non-Abelian quantum field theory." Thesis, University of Liverpool, 2001. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.368626.
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Priebe, Roman. "The regular histories formulation of quantum theory." Thesis, University of Oxford, 2012. http://ora.ox.ac.uk/objects/uuid:937eefeb-35d5-4343-9846-46cc6677ad0c.
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A measurement-independent formulation of quantum mechanics called ‘regular histories’ (RH) is presented, able to reproduce the predictions of the standard formalism without the need to for a quantum-classical divide or the presence of an observer. It applies to closed systems and features no wave-function collapse. Weights are assigned only to histories satisfying a criterion called ‘regularity’. As the set of regular histories is not closed under the Boolean operations this requires a new con- cept of weight, called ‘likelihood’. Remarkably, this single change is enough to overcome many of the well-known obstacles to a sensible interpretation of quantum mechanics. For example, Bell’s theorem, which makes essential use of probabilities, places no constraints on the locality properties of a theory based on likelihoods. Indeed, RH is both counter- factually definite and free from action-at-a-distance. Moreover, in RH the meaningful histories are exactly those that can be witnessed at least in principle. Since it is especially difficult to make sense of the concept of probability for histories whose occurrence is intrinsically indeterminable, this makes likelihoods easier to justify than probabilities. Interaction with the environment causes the kinds of histories relevant at the macroscopic scale of human experience to be witnessable and indeed to generate Boolean algebras of witnessable histories, on which likelihoods reduce to ordinary probabilities. Further- more, a formal notion of inference defined on regular histories satisfies, when restricted to such Boolean algebras, the classical axioms of implication, explaining our perception of a largely classical world. Even in the context of general quantum histories the rules of reasoning in RH are remark- ably intuitive. Classical logic must only be amended to reflect the fundamental premise that one cannot meaningfully talk about the occurrence of unwitnessable histories. Crucially, different histories with the same ‘physical content’ can be interpreted in the same way and independently of the family in which they are expressed. RH thereby rectifies a critical flaw of its inspiration, the consistent histories (CH) approach, which requires either an as yet unknown set selection rule or a paradigm shift towards an un- conventional picture of reality whose elements are histories-with-respect-to-a-framework. It can be argued that RH compares favourably with other proposed interpretations of quantum mechanics in that it resolves the measurement problem while retaining an essentially classical worldview without parallel universes, a framework-dependent reality or action-at-a-distance.
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Gatward, Sally Morrell. "On a new construction in group theory." Thesis, Queen Mary, University of London, 2011. http://qmro.qmul.ac.uk/xmlui/handle/123456789/2342.
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My supervisors Ian Chiswell and Thomas M¨uller have found a new class of groups of functions defined on intervals of the real line, with multiplication defined by analogy with multiplication in free groups. I have extended this idea to functions defined on a densely ordered abelian group. This doesn’t give rise to a class of groups straight away, but using the idea of exponentiation from a paper by Myasnikov, Remeslennikov and Serbin, I have formed another class of groups, in which each group contains a subgroup isomorphic to one of Chiswell and M¨uller’s groups. After the introduction, the second chapter defines the set that contains the group and describes the multiplication for elements within the set. In chapter three I define exponentiation, which leads on to chapter four, in which I describe how it is used to find my groups. Then in chapter five I describe the structure of the centralisers of certain elements within the groups.
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Keil, Markus. "Renormalization group theory for quantum dissipative systems in nonequilibrium." [S.l.] : [s.n.], 2001. http://webdoc.sub.gwdg.de/diss/2002/keil/keil.pdf.
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Li, Ben. "Convex Analysis and its Application to Quantum Information Theory." Case Western Reserve University School of Graduate Studies / OhioLINK, 2018. http://rave.ohiolink.edu/etdc/view?acc_num=case1534240584297621.
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Zhang, Yu, and 張余. "Time-dependent study of quantum transport and dissipation." Thesis, The University of Hong Kong (Pokfulam, Hong Kong), 2014. http://hdl.handle.net/10722/207190.
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Dissipative time-dependent quantum transport theory with electron-phonon interaction in either weak or strong coupling regime is established. This theory goes beyond the conventional quantum master equation method and Kadanoff-Baym kinetic equations. It provides an efficient method for the simulation of transient quantum transport under arbitrary bias voltage with different electron-phonon coupling strength.
First, time-dependent quantum transport theory for non-interacting system and its combination with first-principles method is developed. Based on the Padé expansion to Fermi function, and wide-band limit approximation of lead self-energy, a set of equations of motion is developed for efficient evaluation of density matrix and related quantities. To demonstrate its applicability, this method is employed to study the transient transport through a carbon nanotube based electronic device.
Second, a dissipative time-dependent quantum transport theory is established in the weak electron-phonon coupling regime. In addition to the self-energy caused by leads, a new self-energy is introduced to characterize the dissipative effect induced by electron-phonon interaction. In the weak coupling regime, the lowest order expansion is employed for practical implementation. The corresponding closed set of equations of motion is derived, which provides an efficient and accurate treatment of transient quantum transport with electron-phonon interaction in the weak coupling regime. Numerical examples are demonstrated and its combination with first-principles method is also discussed.
Next, a dissipative quantum transport theory for strong electron-phonon interaction is established by employing small polaron transformation. The corresponding equation of motions are developed, which is used to study the quantum interference effect and phonon-induced decoherence dynamics. Numerical studies demonstrate the formation of quantum interference effect caused by the transport electrons through two quasi-degenerate states with different couplings to the leads. The quantum interference can be suppressed by phonon scattering, which indicates the importance of considering electron-phonon interaction in these systems with prominent quantum interference effect when the electron-phonon coupling is strong.
Last, the dissipative quantum transport theory for weak electron-phonon coupling regime is used to simulate the photovoltaic devices. Within the nonequilibrium Greens function formalism, a quantum mechanical method for nanostructured photovoltaic devices is presented. The method employs density-functional tight-binding theory for electronic structure, which make is possible to simulate the performance of photovoltaic devices without relying on empirical parameters. Numerical studies of silicon nanowirebased devices of realistic sizes with more than ten thousand atoms are performed and the results indicate that atomistic details and nonequilibrium conditions have clear impact on the photoresponse of the devices.published_or_final_version
Chemistry
Doctoral
Doctor of Philosophy
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Patureau-Mirand, Bertrand. "Invariants topologiques quantiques non semi-simples." Habilitation à diriger des recherches, Université de Bretagne Sud, 2012. http://tel.archives-ouvertes.fr/tel-00872405.
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Invariants topologiques quantiques non semi-simples. La théorie des nœuds (courbes simples plongées dans R³, à déformation continue près) se développe au début du XXième siècle avec notamment les travaux d'Alexander et de Reidemeister. Elle a connu un tournant avec la topologie quantique née en 1984 par la découverte par Vaughan Jones d'une manière d'associer à chaque nœuds un polynôme. Vladimir Turaev et Nicolai Reshetikhin interprètent et généralisent ce procédé en terme de représentations des groupes quantiques. Aujourd'hui encore, la compréhension géométrique de ces invariants est ténue. Toujours dans les années 80, Edward Witten donne une interprètation physique du polynôme de Jones et suggère une généralisation aux variétés de dimension trois. Vladimir Turaev avec Nicolai Reshetikhin puis avec Oleg Viro réalise rigoureusement ces invariants nouveaux pour les variétés de dimension trois. Dans de nombreux cas, ces constructions s'avèrent triviales. Ceci est lié à la présence de représentations des groupes quantiques qui ne sont pas semi-simples. Mes travaux, en collaboration avec Nathan Geer, Vladimir Turaev, Francesco Costantino et Alexis Virelizier ont consisté, pour une grande part, à modifier les constructions précédentes pour définir des invariants non triviaux dans ce cadre non semi-simple. Ces travaux m'ont amené a développer, avec Nathan Geer et Jonathan Kujawa, des techniques algébriques qui présentent un intérêt propre en théorie des représentations. Relier les constructions de la topologie quantique et les invariants d'origine plus géométriques constitue un vrai challenge des mathématiques modernes pour lequel les invariants non semi-simples que j'ai définis offrent un point de vue prometteur.
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Kilgour, Calum Wallace. "Using pictures in combinatorial group and semigroup theory." Thesis, University of Glasgow, 1995. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.265965.
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Acharya, Anirudh. "Quantum tomography : asymptotic theory and statistical methodology." Thesis, University of Nottingham, 2018. http://eprints.nottingham.ac.uk/49998/.
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Recent experimental progress in the preparation and control of quantum systems has brought to light the importance of Quantum State Tomography (QST) in validating the results. In this thesis we investigate several aspects of QST, whose central problem is to devise estimation schemes for the recovery of an unknown state, given an ensemble of n independent identically prepared systems. The key issues in tackling QST for large dimensional systems is the construction of physically relevant low dimensional state models, and the design of appropriate measurements. Inspired by compressed sensing tomography, in chapters 4, 5 we consider the statistical problem of estimating low rank states (r ≪ d) in the set-up of Multiple Ions Tomography (MIT), where r and d are the rank and the dimension of the state respectively. We investigate how the estimation error behaves with a reduction in the number of measurement settings, compared to ‘full’ QST in two setups - Pauli and random bases measurement designs. We study the estimation errors in this ‘incomplete’ measurement setup in terms of a concentration of the Fisher information matrix. For the random bases design we demonstrate that O(r logd) settings suffice for the mean square error w.r.t the Frobenius norm to achieve the optimal O(1/n) rate of estimation. When the error functions are locally quadratic, like the Frobenius norm, then the expected error (or risk) of standard procedures achieves this optimal rate. However, for fidelity based errors such as the Bures distance we show that no ‘compressive’ recovery exists for states close to the boundary, and it is known that even with conventional ‘full’ tomography schemes the risk scales as O(1/√n) for such states and error functions. For qubit states this boundary is the surface of the Bloch sphere. Several estimators have been proposed to improve this scaling with ‘adaptive’ tomography. In chapter 6 we analyse this problem from the perspective of the maximum Bures risk over all qubit states. We propose two adaptive estimation strategies, one based on local measurements and another based on collective measurements utilising the results of quantum local asymptotic normality. We demonstrate a scaling of O(1/n) for the maximum Bures risk with both estimation strategies, and also discuss the construction of a minimax optimal estimator. In chapter 7 we return to the MIT setup and systematically compare several tomographic estimators in an extensive simulation study. We present and analyse results from this study, investigating the performance of the estimators across various states, measurement designs and error functions. Along with commonly used estimators like maximum likelihood, we propose and evaluate a few new ones. We finally introduce two web-based applications designed as tools for performing QST simulations online.
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Eyink, Gregory Lawrence. "Quantum field-theory in non-integer dimensions /." The Ohio State University, 1988. http://rave.ohiolink.edu/etdc/view?acc_num=osu1487584612164091.
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Gordon, Iain. "Representation theory of quantised function algebras at roots of unity." Thesis, Connect to electronic version, 1998. http://hdl.handle.net/1905/177.
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Bavuma, Yanga. "Some combinatorial aspects in algebraic topology and geometric group theory." Master's thesis, University of Cape Town, 2018. http://hdl.handle.net/11427/29763.
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The present Msc thesis deals with classical topics of topology and it has been written, referring to [C. Kosniowski, Introduction to Algebraic Topology, Cambridge University Press, 1980, Cambridge], which is a well known textbook of algebraic topology. It has been selected a list of main exercises from this reference, whose solutions were not directly available, or subject to differerent methods. In fact combinatorial methods have been preferred and the result is a self-contained dissertation on the theory of the fundamental group and of the coverings. Finally, there are some recent problems in geometric group theory which are related to the presence of finitely presented groups which appear naturally as fundamental groups.
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Isenrich, Claudio Llosa. "Kähler groups and Geometric Group Theory." Thesis, University of Oxford, 2017. https://ora.ox.ac.uk/objects/uuid:4a7ab097-4de5-4b72-8fd6-41ff8861ffae.
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In this thesis we study Kähler groups and their connections to Geometric Group Theory. This work presents substantial progress on three central questions in the field: (1) Which subgroups of direct products of surface groups are Kähler? (2) Which Kähler groups admit a classifying space with finite (n-1)-skeleton but no classifying space with finitely many n-cells? (3) Is it possible to give explicit finite presentations for any of the groups constructed in response to Question 2? Question 1 was raised by Delzant and Gromov. Question 2 is intimately related to Question 1: the non-trivial examples of Kähler subgroups of direct products of surface groups never admit a classifying space with finite skeleton. The only known source of non-trivial examples for Questions 1 and 2 are fundamental groups of fibres of holomorphic maps from a direct product of closed surfaces onto an elliptic curve; the first such construction is due to Dimca, Papadima and Suciu. Question 3 was posed by Suciu in the context of these examples. In this thesis we: provide the first constraints on Kähler subdirect products of surface groups (Theorem 7.3.1); develop new construction methods for Kähler groups from maps onto higher-dimensional complex tori (Section 6.1); apply these methods to obtain irreducible examples of Kähler subgroups of direct products of surface groups which arise from maps onto higher-dimensional tori and use them to show that our conditions in Theorem 7.3.1 are minimal (Theorem A); apply our construction methods to produce irreducible examples of Kähler groups that (i) have a classifying space with finite (n-1)-skeleton but no classifying space with finite n-skeleton and (ii) do not have a subgroup of finite index which embeds in a direct product of surface groups (Theorem 8.3.1); provide a new proof of Biswas, Mj and Pancholi's generalisation of Dimca, Papadima and Suciu's construction to more general maps onto elliptic curves (Theorem 4.3.2) and introduce invariants that distinguish many of the groups obtained from this construction (Theorem 4.6.2); and, construct explicit finite presentations for Dimca, Papadima and Suciu's groups thereby answering Question 3 (Theorem 5.4.4)).
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Holland, Jan W. "Properties of the operator product expansion in quantum field theory." Thesis, Cardiff University, 2013. http://orca.cf.ac.uk/53230/.
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We prove that the operator product expansion (OPE), which is usually thought of as an asymptotic short distance expansion, actually converges at arbitrary finite distances within perturbative quantum field theory. The result is derived for the massive scalar field with $\varphi^{4}$-interaction on Euclidean spacetime. This constitutes a generalisation of an earlier result by Hollands and Kopper, which states that the OPE of exactly two quantum fields converges. We also show that the OPE coefficients satisfy factorisation conditions for certain configurations of the spacetime arguments. Such conditions are known to encode information on the algebraic structure of the underlying quantum field theory. Both results rely on modified versions of the renormalisation group flow equations, which allow us to derive explicit bounds on the remainder of these expansions. Within this framework, we also derive a new formula for the perturbation of OPE coefficients, i.e. an equation relating coefficients at a given perturbation order to those of lower order. By iteration of this formula, a new constructive method for the computation of OPE coefficients in perturbation theory is obtained, which only requires the coefficients of the free theory as initial data. Finally, we investigate a strategy to restrict renormalisation ambiguities in quantum field theory via the condition that the OPE coefficients depend analytically on the coupling constant(s) of the respective model. We apply this strategy to the computation of the vacuum expectation value of the stress energy operator in the two dimensional Gross-Neveu model and we obtain a unique prediction for the non-perturbative contribution to this expectation value, which is of the order $\exp(-2\pi/g^{2})$ (here $g$ is the coupling constant). We discuss the possibility that a similar effect, if present in the Standard Model of particle physics, could account for the ''unnatural'' smallness of the cosmological constant.
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Fennessey, Eric James. "Some applications of geometric techniques in combinatorial group theory." Thesis, University of Glasgow, 1989. http://theses.gla.ac.uk/6159/.
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Combinatorial group theory abounds with geometrical techniques. In this thesis we apply some of them to three distinct areas. In Chapter 1 we present all of the techniques and background material neccessary to read chapters 2,3,4. We begin by defining complexes with involutary edges and define coverings of these. We then discuss equivalences between complexes and use these in §§1.3 and 1.4 to give a way (the level method) of simplifying complexes and an application of this method (Theorem 1.3). We then discuss star-complexes of complexes. Next we present background material on diagrams and pictures. The final section in the chapter deals with SQ-universality. The.basic discussion of complexes is taken from notes, by Pride, on complexes without involutary edges, and modified by myself to cover complexes with involution. Chapters 2,3, and 4 are presented in the order that the work for them was done. Chapters 2,3, alld 4 are intended (given the material in chapter 1) to be self contained, and (iv) each has a full introduction. In Chapter 2 we use diagrams and pictures to study groups with the following structure. (a) Let r be a graph with vertex set V and edge set E. We assume that no vertex of r is isolated. (b) For each vertex VEV there is a non-trivial group Gv ' (c) For each edge e-{u,v}EE there is a set Se of cyclically reduced elements of Gu*Gv , each of length at least two. We define Ge to be the quotient of Gu*Gv by the normal closure of Se. We let G be the quotient of *Gv by the normal closure of VEV S- USe. For convenience, we write eEE The above is a generalization ofa situation studied by Pride [35], where each Gv was infinite cyclic.' Let e-{u,v} be an edge of r. We will say that Ge has property-Wk if no non-trivial element of Gu*Gv of free product length less than or equal to 2k is in the kernel of the natural epimorphism (v) We will work with one of the following: (I) Each Ge has property-W2 (II) r is triangle-free and each Ge has property-WI' Assuming that (I) or (II) holds we: (i) prove a Freihietssatz for these groups; (ii) give sufficient conditions for the groups to be SQ-universal; (iii) prove a result which allows us to give long exact sequences relating the (co)-homology G to the (co)-homology of the groups The work in Chapter 2 is in some senses the least original. The proofs are extensions of proofs given in [35] and [39] for the case when each Gv is infinite cyclic. However. there are some technical difficulties which we had to overcome. In chapter 3 we use the two ideas of star-complexes and coverings to look at NEC-groups. An NEC (Non-Euclidean Crystallographic) group is a discontinuous group of isometries (some of which may be (vi) orientation reversing) of the Non-Euclidean plane. According to Yilkie [46], a finitely generated NEC-group with compact orbit space has a presentation as follows: Involutary generators: Yij (i,j)EZo Non-involutary generators: 6i (iElf), tk (l~~r) (*) Defining paths: (YijYij+,)mij (iElf, l~j~n(i)-l) where In Hoare, Karrass and Solitar [22] it is shown that a subgroup of finite index in a group with a presentation of the form (*), has itself a presentation of the form (*). In [22] the same authors show that a subgroup of infinite ingex in a group with a presentation of the form (*) is a free product of groups of the following types: (A) Cyclic groups. (vii) (B) Groups with presentations of the form Xl' ... 'Xn involutary. (e) Groups with presentations of the form Xi (iEZ) involutary. We define what we mean by an NEe-complex. (This involves a structural re$triction on the form of the star-complex of the complex.) It is obvious from the definition that this class of complexes is clo$ed under coverings, so that the class of fundamental groups of NEe-complexes is trivially closed under taking subgroups. We then obtain structure theorems for both finite and infinite NEe-complexes. We show that the fundamental group of a finite NEe-complex has a presentation of the form (*) and that the fundamental group of an infinite NEe-complex is a free product of groups of the forms (A). (B) and (e) above. We then use coverings to derive some of the results on normal subgroups of NEe-groups given in [5] and [6]. , (viii) In chapter 4 we use the techniques of coverings and diagrams. to stue,iy the SQ-universau'ty of Coxeter groups. This is a problem due to B.H. Neumann (unpublished). see [40]. A Coxeter pair is a 2-tup1e (r.~) where r is a graph (with vertex set V(r) and edge set E(r» and ~ is a map from E(r) to {2.3.4 •.•• }. We associate with (r.~) the Coxeter group c(r,~) defined by the presentation tr(r,~)-
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Altschul, Brett David 1977. "Aspects of quantum theory in 1+1 and slightly more dimensions." Thesis, Massachusetts Institute of Technology, 2003. http://hdl.handle.net/1721.1/29343.
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Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Mathematics, 2003.Includes bibliographical references (p. 81-86).
We consider four problems in (1+1)-dimensional physics. Each of these problems had important connections to the physical behavior of (3+ 1 )-dimensional systems. First, we consider problem of fermions interacting with multiple bosonic solitons. We describe a new approximation scheme for determining the fermion energy spectrum and apply it to (1 + 1 )-dimensional two-component fermions coupled to scalar field solitons. Second, we study (1+1)-dimensional behavior in particles falling toward a Schwarzchild black hole . Using a non-covariant choice for the momentum cutoff, we examine the photon self-energy integral. We find evidence that the photons acquire an effective mass with a nonzero imaginary part, so that the photons may decay. Third, we consider cold fermions trapped in a high aspect ratio potential, which confines the particles to move in only one direction. The purely (1 + 1 )-dimensional aspects of this problem have been extensively studied. We examine the corrections that arise because of the underlying (3+ 1 )-dimensional character of the situation and determine the zero-temperature shifts in the (1+1)-dimensional energy spectrum. Fourth, we present a toy model, which is related, by analogy to the problem of electron-inhabited bubbles in liquid helium. An analysis of the I-dimensional model suggests that the recent suggestion that the electron bubbles may split in two is incorrect.
by Brett David Altschul.
Ph.D.
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Ryan, James Patrick. "Coupling matter to quantum gravity in the group field theory approach." Thesis, University of Cambridge, 2007. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.613184.
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Shiri-Garakani, Mohsen. "Finite Quantum Theory of the Harmonic Oscillator." Diss., Georgia Institute of Technology, 2004. http://hdl.handle.net/1853/5078.
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We apply the Segal process of group simplification to the linear harmonic oscillator. The result is a
finite quantum theory with three quantum constants instead of the usual one. We compare the classical (CLHO), quantum (QLHO), and finite (FLHO) linear harmonic oscillators and their canonical or unitary groups. The FLHO is isomorphic to a
dipole rotator with N=l(l+1) states where l is very large for physically interesting case. The position and momentum variables are quantized with uniform finite spectra. For fixed quantum constants and large N there are three broad classes of FLHO: soft, medium, and hard corresponding respectively to cases where ratio of the of potential energy to kinetic energy in the Hamiltonian is very small, almost equal to one, or very large
The field oscillators responsible for infra-red and
ultraviolet divergences are soft and hard respectively. Medium oscillators approximate the QLHO. Their
low-lying states have nearly the same zero-point
energy and level spacing as the QLHO, and nearly obeying the Heisenberg uncertainty principle and the equipartition principle. The corresponding rotators are nearly polarized along the z-axis.
The soft and hard FLHO's have infinitesimal
0-point energy and grossly violate equipartition and the Heisenberg uncertainty principle. They do not resemble the QLHO at all. Their low-lying energy states correspond to rotators polaroizd along x-axis or y-axis respectively. Soft oscillators have
frozen momentum, because their maximum potential energy is too small to produce one quantum of momentum. Hard oscillators have frozen position, because their maximum kinetic energy is too small to produce one quantum of momentum. Hard oscillators have frozen position, because their maximum kinetic energy is too small to excite one quantum of position.
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Bar, Krzysztof. "Automated rewriting for higher categories and applications to quantum theory." Thesis, University of Oxford, 2016. https://ora.ox.ac.uk/objects/uuid:ba1d3341-873d-4255-8400-c2277b7648f3.
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The contribution of this thesis is a novel framework for rewriting in higher categories. Its theoretical foundation is the theory of quasistrict higher categories and the practical realisation is a proof assistant Globular. The framework introduces the notions of diagrams and signatures as new mutually-recursive structures that give the algebraic basis for the approach. These structures are related the notion of an n-polygraph, but allow reasoning about quasistrict higher categorical structures in a way amenable to computer implementation. Building on this, we propose a new definition of a quasistrict 4-category, and prove a result that in a quasistrict 4-category, an adjunction of 1-morphisms gives rise to a coherent adjunction satisfying the butterfly equations.
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Jacobs, Andrew D. "Nonstandard quantum groups : twisting constructions and noncommutative differential geometry." Thesis, University of St Andrews, 1998. http://hdl.handle.net/10023/13693.
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The general subject of this thesis is quantum groups. The major original results are obtained in the particular areas of twisting constructions and noncommutative differential geometry. Chapters 1 and 2 are intended to explain to the reader what are quantum groups. They are written in the form of a series of linked results and definitions. Chapter 1 reviews the theory of Lie algebras and Lie groups, focusing attention in particular on the classical Lie algebras and groups. Though none of the quoted results are due to the author, such a review, aimed specifically at setting up the paradigm which provides essential guidance in the theory of quantum groups, does not seem to have appeared already. In Chapter 2 the elements of the quantum group theory are recalled. Once again, almost none of the results are due to the author, though in Section 2.10, some results concerning the nonstandard Jordanian group are presented, by way of a worked example, which have not been published. Chapter 3 concerns twisting constructions. We introduce a new class of 2-cocycles defined explicitly on the generators of certain multiparameter standard quantum groups. These allow us, through the process of twisting the familiar standard quantum groups, to generate new as well as previously known examples of non-standard quantum groups. In particular we are able to construct generalisations of both the Cremmer-Gervais deformation of SL(3) and the so called esoteric quantum groups of Fronsdal and Galindo in an explicit and straightforward manner. In Chapter 4 we consider the differential calculus on Hopf algebras as introduced by Woronowicz. We classify all 4-dimensional first order bicovariant calculi on the Jordanian quantum group GL[sub]h,[sub]g(2) and all 3-dimensional first order bicovariant calculi on the Jordanian quantum group SL[sub]h(2). In both cases we assume that the bicovariant bimodules are generated as left modules by the differentials of the quantum group generators. It is found that there are 3 1-parameter families of 4-dimensional bicovariant first order calculi on GL[sub]h,[sub]g(2) and that there is a single, unique, 3-dimensional bicovariant calculus on SL[sub]h(2). This 3-dimensional calculus may be obtained through a classical-like reduction from any one of the three families of 4-dimensional calculi on GL[sub]h,[sub]g(2). Details of the higher order calculi and also the quantum Lie algebras are presented for all calculi. The quantum Lie algebra obtained from the bicovariant calculus on SL[sub]h(2) is shown to be isomorphic to the quantum Lie algebra we obtain as an ad-submodule within the Jordanian universal enveloping algebra U[sub]h(sl[sub]2(C)) and also through a consideration of the decomposition of the tensor product of two copies of the deformed adjoint module. We also obtain the quantum Killing form for this quantum Lie algebra.
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Siehler, Jacob A. "Near-Group Categories." Diss., Virginia Tech, 2003. http://hdl.handle.net/10919/26962.
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We consider the possibility of semisimple tensor categories whose fusion rule includes exactly one noninvertible simple object, so-called near-group categories. Data describing the fusion rule is reduced to an abelian group G and a nonnegative integer k. Conditions are given, in terms of G and k, for the existence or nonexistence of coherent associative structures for such fusion rules (ie, solutions to MacLane's pentagon equation). An explicit construction of matrix solutions to the pentagon equations is given for the cases where we establish existence, and classification of the distinct solutions is carried out partially. Many of these associative structures also support (braided) commutative and tortile structures and we indicate when the additional structures are possible. Small examples are presented in detail suitable for use in computational applications.Ph. D.
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Yeo, Michelle SoYeong. "CONSTRUCTION OF FINITE GROUP." CSUSB ScholarWorks, 2017. https://scholarworks.lib.csusb.edu/etd/592.
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The main goal of this project is to present my investigation of finite images of the progenitor 2^(*n) : N for various N and several values of n. We construct each image by using the technique of double coset enumeration and give a proof of the isomorphism type of the image. We obtain the group 7^2: D_6 as a homomorphic image of the progenitor 2^(*14) : D_14, we obtain the group 2^4 : (5 : 4) as a homomorphic image of the progenitor 2^(*5) : (5 : 4), we obtain the group (10 x10) : ((3 x 4) : 2) as a homomorphic image of the progenitor 2^(*15) : (15x4), we obtain the group PGL(2; 7) as a homomorphic image of the progenitor 2^7 : D_14, we obtain the group S_6 as a homomorphic image of the progenitor 2^5 : (5 : 4), and we obtain the group S_7 as a homomorphic image of the progenitor 2^(*15) : (15 : 4). Also, have given some unsuccessful progenitors.
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Vaintrob, Dmitry. "Mirror symmetry and the K theory of a p-adic group." Thesis, Massachusetts Institute of Technology, 2016. http://hdl.handle.net/1721.1/104578.
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Thesis: Ph. D., Massachusetts Institute of Technology, Department of Mathematics, 2016.Cataloged from PDF version of thesis.
Includes bibliographical references (pages 59-61).
Let G be a split, semisimple p-adic group. We construct a derived localization functor Loc : ... from the compactified category of [BK2] associated to G to the category of equivariant sheaves on the Bruhat-Tits building whose stalks have finite-multiplicity isotypic components as representations of the stabilizer. Our construction is motivated by the "coherent-constructible correspondence" functor in toric mirror symmetry and a construction of [CCC]. We show that Loc has a number of useful properties, including the fact that the sections ... compactifying the finitely-generated representation V. We also construct a depth by Dmitry A. Vaintrob.
Ph. D.
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Joubert, Paul. "Geometric actions of the absolute Galois group." Thesis, Stellenbosch : University of Stellenbosch, 2006. http://hdl.handle.net/10019.1/2508.
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Thesis (MSc (Mathematics))--University of Stellenbosch, 2006.This thesis gives an introduction to some of the ideas originating from A. Grothendieck's 1984 manuscript Esquisse d'un programme. Most of these ideas are related to a new geometric approach to studying the absolute Galois group over the rationals by considering its action on certain geometric objects such as dessins d'enfants (called stick figures in this thesis) and the fundamental groups of certain moduli spaces of curves. I start by defining stick figures and explaining the connection between these innocent combinatorial objects and the absolute Galois group. I then proceed to give some background on moduli spaces. This involves describing how Teichmuller spaces and mapping class groups can be used to address the problem of counting the possible complex structures on a compact surface. In the last chapter I show how this relates to the absolute Galois group by giving an explicit description of the action of the absolute Galois group on the fundamental group of a particularly simple moduli space. I end by showing how this description was used by Y. Ihara to prove that the absolute Galois group is contained in the Grothendieck-Teichmuller group.
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Ketcham, Kwang B. "Group Frames and Partially Ranked Data." Scholarship @ Claremont, 2010. https://scholarship.claremont.edu/hmc_theses/19.
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We give an overview of finite group frames and their applications to calculating summary statistics from partially ranked data, drawing upon the work of Rachel Cranfill (2009). We also provide a summary of the representation theory of compact Lie groups. We introduce both of these concepts as possible avenues beyond finite group representations, and also to suggest exploration into calculating summary statistics on Hilbert spaces using representations of Lie groups acting upon those spaces.
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Christandl, Matthias. "The structure of bipartite quantum states : insights from group theory and cryptography." Thesis, University of Cambridge, 2006. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.613714.
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Roy, Alan A. "Aspects of renormalisation in some quantum field theories." Thesis, Rhodes University, 1998. http://hdl.handle.net/10962/d1005214.
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Renormalisation is an important aspect of Quantum Field Theory. It is used to create physically meaningful theories and some major developments took place in the 1970's and onwards. We consider Renormalisation in its application to the theories of ψ⁴ , Quantum Electrodynamics, Quantum Chromodynamics and the Background Field Method. Feynman diagrams are used to illustrate many of the concepts.
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Silberstein, Aaron. "Anabelian Intersection Theory." Thesis, Harvard University, 2012. http://dissertations.umi.com/gsas.harvard:10141.
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Let F be a field finitely generated and of transcendence degree 2 over \(\bar{\mathbb{Q}}\). We describe a correspondence between the smooth algebraic surfaces X defined over \(\bar{\mathbb{Q}}\) with field of rational functions F and Florian Pop’s geometric sets of prime divisors on \(Gal(\bar{F}/F)\), which are purely group-theoretical objects. This allows us to give a strong anabelian theorem for these surfaces. As a corollary, for each number field K, we give a method to construct infinitely many profinite groups \(\Gamma\) such that \(Out_{cont} (\Gamma)\) is isomorphic to \(Gal(\bar{K}/K)\), and we find a host of new categories which answer the Question of Ihara/Conjecture of Oda-Matsumura.Mathematics
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Davis, Simon. "The quantum cosmological wavefunction at very early times for a quadratic gravity theory." Universität Potsdam, 2003. http://opus.kobv.de/ubp/volltexte/2008/2652/.
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The quantum cosmological wavefunction for a quadratic gravity theory derived from the heterotic string effective action is obtained near the inflationary epoch and during the initial Planck era. Neglecting derivatives with respect to the scalar field, the wavefunction would satisfy a third-order differential equation near the inflationary epoch which has a solution that is singular in the scale factor limit a(t) → 0. When scalar field derivatives are included, a sixth-order differential equation is obtained for the wavefunction and the solution by Mellin transform is regular in the a → 0 limit. It follows that inclusion of the scalar field in the quadratic gravity action is necessary for consistency of the quantum cosmology of the theory at very early times.
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Michlin, Tracie L. "Using wavelet bases to separate scales in quantum field theory." Diss., University of Iowa, 2017. https://ir.uiowa.edu/etd/5572.
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This thesis investigates the use of Daubechies wavelets to separate scales in local quantum field theory. Field theories have an infinite number of degrees of freedom on all distance scales. Quantum field theories are believed to describe the physics of subatomic particles. These theories have no known mathematically convergent approximation methods. Daubechies wavelet bases can be used separate degrees of freedom on different distance scales. Volume and resolution truncations lead to mathematically well-defined truncated theories that can be treated using established methods. This work demonstrates that flow equation methods can be used to block diagonalize truncated field theoretic Hamiltonians by scale. This eliminates the fine scale degrees of freedom. This may lead to approximation methods and provide an understanding of how to formulate well-defined fine resolution limits.
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Alp, Murat. "GAP, crossed inodules, Cat'1-groups : applications of computational group theory." Thesis, Bangor University, 1997. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.361168.
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Chen, Shuguang, and 陈曙光. "Nonequilibrium Green's function-hierarchical equation of motion method for time-dependent quantum transport." Thesis, The University of Hong Kong (Pokfulam, Hong Kong), 2014. http://hdl.handle.net/10722/206344.
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The nonequilibrium Green’s function-hierarchical equation of motion (NEGFHEOM) method has been developed to simulate the time-dependent electron transport process. The real-time evolution of the reduced single-electron density matrix is solved through the Liouville-von-Neumann equation. The method is very efficient compared to conventional NEGF formulas which need to discretize the simulation time. The hierarchical equation of motion (HEOM) is closed at the second-tier in the time-dependent noninteracting Kohn-Sham framework. When combined with the wide band limit (WBL) approximation, the HEOM terminate at the first-tier. The resulting NEGF-HEOM-WBL method is particularly suitable for simulating the long time transient dynamics for large systems.
The method developed is first applied to calculate the transient current through an array of as many as 1000 quantum dots. Upon switching on the bias voltage, the current increases linearly with respect to time before reaching its steady state value. And the time required for the current to reach its steady state value is exactly the time for a conducting electron to travel through the array at Fermi velocity. These phenomena can be understood by simple analysis on the energetics of the quantum dots or by classical electron gas model.
Then the method is employed to investigate several simple molecular circuits, in which the para-linkage or meta-linkage benzene acts as the transmitting molecular entity. The simulation results shows that it takes a certain amount of time before the quantum interference manifests itself, and that the transmission through the meta case is hundreds of times smaller than that through the para case. To investigate the quantum interference process in molecular electronics, the concept of Büttiker probe is introduced. The Büttiker probe is an electrode that, when attached to electronic devices, causes the coherence passing through disappear. Simulation results show that the Büttiker probe can enhance the transmission of the meta benzene system through destroying the constructive interference. By turning the probe on and off, it can be observed that large strong correlations are indeed built up as electrons are transported through benzenoid structures - when the decoherence is turned off, the current rises, and when the decoherence is turned back on, the current falls.
Finally, TDDFT(B)-NEGF-HEOM-WBL method is implemented to solve realistic systems in the formalism of time-dependent density functional theory (tightbinding). Ab initio calculations are carried out to simulate the time-dependent electron transport through a CNT-based device. The simulation results show that when the input bias voltage is in low frequency, both the conventional adiabatic approximation method and the NEGF-HEOM-WBL methods are good enough. However, when high frequency dynamic responses are need to be captured, the NEGF-HEOM-WBL method is more suitable.published_or_final_version
Chemistry
Doctoral
Doctor of Philosophy
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Cornwell, Christopher R. "On the Combinatorics of Certain Garside Semigroups." Diss., CLICK HERE for online access, 2006. http://contentdm.lib.byu.edu/ETD/image/etd1381.pdf.
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