Nanowhiskers semicondutores de compostos III-V apresentam grande potencial para aplicações tecnológicas. Controlando as condições de crescimento, tais como temperatura e diâmetro, é possível alternar entre as fases cristalinas zincblend e wurtzita, dando origem ao politipismo. Esse efeito tem grande influência nas propriedades eletrônicas e óticas do sistema, gerando novas formas de confinamento para os portadores. Um modelo teórico capaz de descrever com exatidão as propriedades eletrônicas e óticas presentes nessas nanoestruturas politípicas pode ser utilizado para o estudo e desenvolvimento de novos tipos de nanodispositivos. Neste trabalho, apresento a construção do Hamiltoniano k.p no ponto Γ para as estruturas cristalinas zincblend e wurtzita baseada no formalismo da teoria de grupos. Utilizando o grupo de simetria do ponto Γ, é possível obter as representações irredutíveis das bandas de energia, partindo de orbitais atômicos e do número de átomos na célula primitiva unitária. Além disso, as operações de simetria do grupo são utilizadas para calcular os elementos de matriz não nulos e independentes do Hamiltoniano k.p. O estudo da simetria dos estados de base pertencentes às representações irredutíveis das bandas de energia, juntamente com a aproximação da função envelope, permitiu a formulação de um modelo polítipico wurtzita/zincblend para cálculo da estrutura de bandas em nanowhiskers. Embora o interesse seja em super-redes politípicas, o modelo proposto foi aplicado a um poço quântico de InP com o intuito de extrair a física envolvida na interface wurtzita/zincblend. / Semiconductor nanowhiskers made of III-V compounds exhibit great potential for technological applications. Controlling the growth conditions, such as temperature and diameter, it is possible to alternate between zincblend and wurtzite crystalline phases, giving origin to the polytypism. This effect has great influence in the electronic and optical properties of the system, generating new forms of confinement to the carriers. A theoretical model capable to accurately describe electronic and optical properties in these polytypical nanostructures can be used to study and develop new kinds of nanodevices. In this study, I present the development of the k.p Hamiltonian in the Γ point for the zincblend and wurtzite crystal structures based on the formalism of group theory. Using the symmetry group of the Γ point, it is possible to obtain the irreducible representations of the energy bands, considering the atomic orbitals and the number of atoms in the primitive unit cell. Also, the group symmetry operations are used to calculate the non-zero and independent matrix elements of the k.p Hamiltonian. The study of the basis states symmetry of irreducible representations in the energy bands, alongside with the envelope function approximation, allowed the formulation of a wurtzite/zincblend polytypical model to calculate the electronic band structure of nanowhiskers. Although the interest is in polytypical superlattices, the proposed model was applied to a single quantum well of InP to extract the physics of the wurtzite/zincblend interface.

In this thesis, we construct an algebraic framework for analyzing committee elections. In this framework, module homomorphisms are used to model positional voting procedures. Using the action of the wreath product group S2[Sn] on these modules, we obtain module decompositions which help us to gain an understanding of the module homomorphism. We use these decompositions to construct some interesting voting paradoxes.

The goal of this presentation is to find original symmetric presentations of finite groups. It is frequently the case, that progenitors factored by appropriate relations produce simple and even sporadic groups as homomorphic images. We have discovered two of the twenty-six sporadic simple groups namely, M12, J1 and the Lie type group Suz(8). In addition several linear and classical groups will also be presented. We will present several progenitors including: 2*12: 22 x (3 : 2), 2*11: PSL2(11), 2*5: (5 : 4) which have produced the homomorphic images: M12 : 2, Suz(8) x 2, and J1 x 2. We will give monomial progenitors whose homomorphic images are: 17*10 :m PGL2(9), 3*4:m Z2 ≀D4 , and 13*2:m (22 x 3) : 2 which produce the homomorphic images:132 : ((2 x 13) : (2 x 3)), 2 x S9, and (22)•PGL4(3). Once we have a presentation of a group we can verify the group's existence through double coset enumeration. We will give proofs of isomorphism types of the presented images: S3 x PGL2(7) x S5, 28:A5, and 2•U4(2):2.

The foundation of the work of this thesis is based around the involutory progenitor and the finite homomorphic images found therein. This process is developed by Robert T. Curtis and he defines it as 2^{*n} :N {pi w | pi in N, w} where 2^{*n} denotes a free product of n copies of the cyclic group of order 2 generated by involutions. We repeat this process with different control groups and a different array of possible relations to discover interesting groups, such as sporadic, linear, or unitary groups, to name a few. Predominantly this work was produced from transitive groups in 6,10,12, and 18 letters. Which led to identify some appealing groups for this project, such as Janko group J1, Symplectic groups S(4,3) and S(6,2), Mathieu group M12 and some linear groups such as PGL2(7) and L2(11) . With this information, we performed double coset enumeration on some of our findings, M12 over L_2(11) and L_2(31) over D15. We will also prove their isomorphism types with the help of the Jordan-Holder theorem, which aids us in defining the make up of the group. Some examples that we will encounter are the extensions of L_2(31)(center) 2 and A5:2^2.

This thesis is on the representation theory of finite groups. Specifically, it is about finding connections between fusion and universal deformation rings. Two elements of a subgroup N of a finite group Γ are said to be fused if they are conjugate in Γ, but not in N. The study of fusion arises in trying to relate the local structure of Γ (for example, its subgroups and their embeddings) to the global structure of Γ (for example, its normal subgroups, quotient groups, conjugacy classes). Fusion is also important to understand the representation theory of Γ (for example, through the formula for the induction of a character from N to Γ). Universal deformation rings of irreducible mod p representations of Γcan be viewed as providing a universal generalization of the Brauer character theory of these mod p representations of Γ. It is the aim of this thesis to connect fusion to this universal generalization by considering the case when Γ is an extension of a finite group G of order prime to p by an elementary abelian p-group N of rank 2. We obtain a complete answer in the case when G is a dihedral group, and we also consider the case when G is abelian. On the way, we compute for many absolutely irreducible FpΓ-modules V, the cohomology groups H2(Γ,HomFp(V,V) for i = 1, 2, and also the universal deformation rings R(Γ,V).

The 2-groups of coclass 1 are widely known and James (in 1975) looked at the 2-groups of coclass 2. Development of the p-group generation algorithm implemented by O'Brien at ANU enabled group presentations to be provided for the 2-groups of coclass 3 by Newman and O'Brien for groups of order up to 223. Newman and O'Brien (in 1999) conjectured the number of descendants of 2n for all n. They introduced the concept of a family, with each family related to a different pro-p-group and the concept of a sporadic p-group, a p-group external to any family. They found 1782 sporadic 2-groups with order at most 214. The 70 families of 2-groups of coclass 3 can be further split according to their period, a measure of the repetitive structure of the families. Newman and O'Brien conjectured that these families had periods of 1, 2 or 4. This thesis examines the 2-groups of coclass 3 contained in families with period 1 and shows that the number of descendants conjectured by Newman and O'Brien is correct. Furthermore the presentation of all groups contained in period 1 families is provided and shown to be correct.

The process of coset enumeration has become a significant factor in group theoretical investigations since the advent of modern computing power, but in some respects the process is still not well understood. This thesis investigates some features of coset enumeration, working mainly with the group F(2,7). Chapter 1 describes the characteristics of coset enumeration and algorithms used for it. A worked example of the method is provided. Chapter 2 discusses some features which would be desirable in computer programs for use in investigating the coset enumeration process itself, and reviews the Havas/Alford program which to date best meets the requirements. Chapter 3 deals with the use of coset ammeration in proofs, either in its own right or as a basis for other workings. An example of one attempt to obtain a proof by coset enumeration is given. Chapter 4 reviews techniques designed to reduce the length of coset enumerations and proposes the 'equality list' technique as a way to reduce enumeration length for some groups. Extra insights obtainable using the equality list method are also discussed. Chapter 5 summarises the factors by which the success of different coset enumerations can be compared and proposes an algorithm for making systematic comparisons among enumerations. Chapter 6 reports five coset enumerations, obtained manually by three main methods on the group F(2,7). All these enumerations were shorter than is so far obtainable by machine and one is shorter than other known hand enumerations. The enumerations were compared by applying the process developed in Chapter 5. Chapter 7 presents a shorter proof of the cyclicity of the group F(2,7) than was hitherto available. The proof derives from the workings for one of the coset enumerations described in Chapter 6. There are eight appendices and an annotated bibliography. The appendices contain, inter alia, edited correspondence between well-known coset-enumerators, a guide to the Havas/Alford program, further details on the equality list method and listings of various enumerations.

Le résultat principal est une caractérisation homotopique des orbifolds de dimension 3 qui sont fibrés de Seifert : si O est un orbifold de dimension 3 fermé, orientable et petit dont le groupe fondamental admet un sous-groupe infini cyclique normal, alors O est de Seifert. Ce théorème généralise un résultat de Scott, Mess, Tukia, Gabai et Casson-Jungreis pour les variétés. Il repose sur une caractérisation des groupes de surfaces virtuels comme groupes quasi-isométriques à un plan riemannien complet. D'autres résultats sur les quasi-isométries entre groupes et surfaces sont obtenus.

The work is divided into three main parts. In a first time (chapter 1) we define a “BTZ” black hole in anti de Sitter space in any dimension. That will be done by means of group theoretical and symmetric spaces considerations. A physical “good domain” is identified as an open orbit of a subgroup of the isometry group of anti de Sitter. Then (chapter 2) we show that the open orbit is in fact isomorphic to a group (we introduce the notion of globally group type manifold) for which a quantization exists. The quantization of the black hole is performed and its Dirac operator is computed. The third part (appendix A and B) exposes some previously known results. Appendix A is given in a pedagogical purpose: it exposes generalities about deformation quantization and careful examples with SL(2,R), and split extensions of Heisenberg algebras. Appendix B is devoted to some classical results about homogeneous spaces and Iwasawa decompositions. Explicit decompositions are given for every algebra that will be used in the thesis. It serves to make the whole text more self contained and to fix notations. Basics of quantization by group action are given in appendix A.4. One more chapter is inserted (chapter 3). It contains two small results which have no true interest by themselves but which raise questions and call for further development. We discuss a product on the half-plane or, equivalently, on the Iwasawa subgroup of SL(2,R), due to A. Unterberger. We show that the quantization by group action machinery can be applied to this product in order to deform the dual of the Lie algebra of that Iwasawa subgroup. Although this result seems promising, we show by two examples that the product is not universal in the sense that even the product of compactly supported functions cannot be defined on AdS2 by the quantization induced by Unterberger's product. Then we show that the Iwasawa subgroup of SO(2,n) (i.e. the group which defines the singularity) is a symplectic split extension of the Iwasawa subgroup of SU(1,1) by the Iwasawa subgroup of SU(1,n). A quantization of the two latter groups being known, a quantization of SO(2,n) is in principle possible using an extension lemma. Properties of this product and the resulting quantization of AdSl were not investigated because we found a more economical way to quantize AdS4 .

Cette thèse est consacrée à l'étude du groupe d'automorphismes de groupes agissant sur des arbres d'une part, et du rang des groupes de Coxeter d'autre part. Via la théorie de Bass-Serre, un groupe agissant sur un arbre est doté d'une structure algébrique particulière, généralisant produits amalgamés et extensions HNN. Le groupe est en fait déterminé par certaines données combinatoires découlant de cette action, appelées graphes de groupes. Un cas particulier de cette situation est celle d'un produit libre. Une présentation du groupe d'automorphisme d'un produit libre d'un nombre fini de groupes librement indécomposables en termes de présentation des facteurs et de leurs groupes d'automorphismes a été donnée par Fouxe-Rabinovich. Il découle de son travail que si les facteurs et leurs groupes d'automorphismes sont de présentation finie, alors le groupe d'automorphisme du produit libre est de présentation finie. Une première partie de cette thèse donne une nouvelle preuve de ce résultat, se basant sur le langage des actions de groupes sur les arbres. Un groupe accessible est un groupe de type fini déterminé par un graphe de groupe fini dont les groupes d'arêtes sont finis et les groupes de sommets ont au plus un bout, c'est-à-dire qu'ils ne se décomposent pas en produit amalgamé ni en extension HNN sur un groupe fini. L'étude du groupe d'automorphisme d'un groupe accessible est ramenée à l'étude de groupes d'automorphismes de produits libres, de groupes de twists de Dehn et de groupes d'automorphismes relatifs des groupes de sommets. En particulier, on déduit un critère naturel pour que le groupe d'automorphismes d'un groupe accessible soit de présentation finie, et on donne une caractérisation des groupes accessibles dont le groupe d'automorphisme externe est fini. Appliqués aux groupes hyperboliques de Gromov, ces résultats permettent d'affirmer que le groupe d'automorphismes d'un groupe hyperbolique est de présentation finie, et donnent une caractérisation précise des groupes hyperboliques dont le groupe d'automorphisme externe est fini. Enfin, on étudie le rang des groupes de Coxeter, c'est-à-dire le cardinal minimal d'un ensemble générateur pour un groupe de Coxeter donné. Plus précisément, on montre que si les composantes de la matrice de Coxeter déterminant un groupe de Coxeter sont suffisamment grandes, alors l'ensemble générateur standard est de cardinal minimal parmi tous les ensembles générateurs.