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.

Deformation theory studies the behavior of mathematical objects, such as representations or modules, under small perturbations. This theory is useful in both pure and applied mathematics and has been used in the proof of many long-standing problems. In particular, in number theory Wiles and Taylor used universal deformation rings of Galois representations in the proof of Fermat's Last Theorem. The main motivation for determining universal deformation rings of modules for finite dimensional algebras is that deep results from representation theory can be used to arrive at a better understanding of deformation rings. In this thesis, I study the universal deformation rings of certain modules for algebras of dihedral type of polynomial growth which have been completely classied by Erdmann and Skowronski using quivers and relations. More precisely, let κ be an algebraically closed field and let λ be a κ-algebra of dihedral type which is of polynomial growth. In this thesis, first classify all λ-modules whose stable endomorphism ring is isomorphic to κ and which are given combinatorially by strings, and then I determine the universal deformation ring of each of these modules.

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).

Ce travail porte sur l'étude de séries financières à l'aide de processus multifractals et notamment de processus MRW (Multifractal Random Walk), introduits par Bacry, Delour et Muzy. Dans ce contexte, on aborde la problématique des événements extrêmes, de l'approximation limite de petite intermittence et de l'estimation statistique des paramètres du modèle MRW log-normal. Les résultats obtenus permettent l'utilisation du modèle MRW pour la prédiction du risque (prédiction de volatilité conditionnelle et de Valeur-à-Risque conditionnelle). Une dernière partie plus exploratoire propose une modélisation des séries financières intra-journalières, modélisation compatible avec l'approche multifractale et permettant d'améliorer la prédiction de risque. Les résultats numériq! ues obtenus sur des données réelles montrent que le modµele MRW log-normal fournit des prédictions de risque de bien meilleure qualité que celles obtenues à l'aide de modèles économétriques plus classiques (GARCH et tGARCH).

Dans cette thèse on étudie différents problèmes portant sur les matroïdes et les matroïdes orientés. On s'intéresse à trois sujets particuliers : la décomposition du polytope des bases d'un matroïde, l'orientation de matroïdes et le jeu de commutation de Shannon. Plus précisément dans le chapitre 2 nous étudions une décomposition spéciale introduite par Lafforgue. Pour un matroïde M, une décomposition du polytope des bases d'un matroïde P(M) est une décomposition de la forme P(M) = St i=1 P(Mi) où chaque P(Mi) est également un polytope des bases d'un matroïde pour un certain matroïde Mi, et pour chaque 1 i 6= j t, l'intersection P(Mi) \ P(Mj) est une face de P(Mi) et de P(Mj). Dans cette thèse, nous étudions la séparation par hyperplan, autrement dit la décomposition du polytope quand t = 2. Nous donnons des conditions suffisantes sur M pour que P(M) puisse avoir une séparation par hyperplan. Nous caractérisons également les cas où P(M1 M2) a une séparation par hyperplan où M1 M2 dénote la somme directe des matroïdes M1 et M2. Nous montrons finalement que P(M) n'a pas de séparation par hyperplan si M est binaire. Dans le chapitre 3 nous étudions la classe des matroïdes orientés du réseau. Après avoir donné une caractérisation complète des matroïdes orientés du réseau en fonction de l'union de matroïdes orientés uniformes de rang un, nous montrons que cette classe est fermée par dualité et par mineurs. Nous étudions ensuite les simplexes de l'arrangement d'hyperplans découlant de matroïdes orientés du réseau. Nous présentons une caractérisation de ces simplexes et construisons un arrangement de n hyperplans en dimension d contenant O(2k(n k )k) simplexes avec n < k = bd 2 c. Nous approfondissons une question posée par Grünbaum [Grünbaum, 1971] concernant les colorations des arrangements de pseudodroites. Nous prolongeons la question de Grünbaum à des arrangements d'hyperplans et répondons par l'affirmative à cette question généralisée pour les arrangements découlants de matroïdes orientés du réseau. Dans le chapitre 4 nous nous sommes intéressés à une une version sur les matroïdes orientés du célèbre jeu de commutation de Shannon, version introduite par Y.O. Hamidoune et M.Las Vergnas[Hamidoune et Las Vergnas, 1997a] en 1986. Ils ont conjecturé que la classification du jeu de commutation sur les matroïdes orientés est identique à la classification de la version non orientée. Dans cette thèse, nous confortons cette conjecture en montrant sa validité pour la classe infinie de matroïdes orientés obtenues comme union de matroïdes orientés uniformes de rang 1 et/ou de rang 2.

Root systems are a fundamental concept in the theory of Lie algebra. In this thesis, we will use two different kind of graphs to represent the group generated by reflections acting on the elements of the root system. The root systems we are interested in are those of type A2, B2 and G2. After drawing the graphs, we will study the algebraic groups corresponding to those root systems. We will use three different techniques to give a geometric description of the homogeneous spaces G/P where G is the algebraic group corresponding to the root system and P is one of its parabolic subgroup. Finally, we will make a link between the graphs and the multiplication of basis elements in the Chow group CH(G/P).

EXPLORING IT-BASED KNOWLEDGE SHARING PRACTICES: REPRESENTING KNOWLEDGE WITHIN AND ACROSS PROJECTS Drawing on the social representation literature combined with a need to better understand knowledge sharing across projects, this research lays the ground for the development of a theoretical account seeking to explain the relationship between project members’ representations of knowledge sharing practices and the use of knowledge-based systems as boundary objects or shared systems. The concept of social representations is particularly appropriate for studying social issues in continuous evolution such as the adoption of a new information system. The research design is structured as an interpretive case study, focusing on the knowledge sharing practices within and across four project groups. The findings showed significant divergence among the groups’ social representations. Sharing knowledge across projects was rather challenging, despite the potential advantages provided by the knowledge-based system. Therefore, technological change does not automatically trigger the intended changes in work practices and routines. The groups’ social representations need to be aligned with the desired behaviour or patterns of actions.

It is impossible to overstate the importance of symmetry in physics and mathematics. Symmetry arguments play a central role in a broad range of problems from simplifying a system of linear equations to a deep role in organizing the fundamental principles of physics. They are used, for instance, in Noether’s theorem to find the consequences of symmetry of a dynamics. For many systems of interest, the dynamics are sufficiently complicated that one cannot hope to characterize their evolution completely, whereas by appealing to the symmetries of the dynamical laws one can easily infer many useful results. In part I of this thesis we study the problem of finding the consequences of symmetry of a (possibly open) dynamics from an information-theoretic perspective. The study of this problem naturally leads us to the notion of asymmetry of quantum states. The asymmetry of a state relative to some symmetry group specifies how and to what extent the given symmetry is broken by the state. Characterizing these is found to be surprisingly useful to constrain which final states of the system can be reached from a given initial state. Another motivation for the study of asymmetry comes from the field of quantum metrology and relatedly the field of quantum reference frames. It turns out that the degree of success one can achieve in many metrological tasks depends only on the asymmetry properties of the state used for metrology. We show that some ideas and tools developed in the field of quantum information theory are extremely useful to study the notion of asymmetry of states and therefore to find the consequences of symmetry of an open or closed system dynamics. In part II of this thesis we present a novel application of symmetry arguments in the field of quantum estimation theory. We consider a family of multi-copy estimation problems wherein one is given n copies of an unknown quantum state according to some prior distribution and the goal is to estimate certain parameters of the given state. In particular, we are interested to know whether collective measurements are useful and if so to find an upper bound on the amount of entanglement which is required to achieve the optimal estimation. We introduce a new approach to this problem by considering the symmetries of the prior and the symmetries of the parameters to be estimated. We show that based on these symmetries one can find strong constraints on the amount of entanglement required to implement the optimal measurement. In order to infer properties of the optimal estimation procedure from the symmetries of the parameters and the prior we come up with a generalization of Schur-Weyl duality. Just as Schur-Weyl duality has many applications to quantum information theory and quantum algorithms so too does this generalization. In this thesis we explore some of these applications.

This qualitative study examined the social representations of work values among nine Swedish students of Generation Y. As the oldest working generation retires, the labor market requires a major adjustment. To facilitate the transition of Generation Y into the labor market requires employers’ knowledge of young peoples’ work values and vision of working life. Strong common values between participants were explained by social representations, which mean they had collective perceptions that were based on the similar influences that affect them as a group and have affected them during childhood. This qualitative study examined nine students who study at Jönköping University and are part of this new working generation. The result was generated by doing semi-structured interviews that was explored by a thematic analysis. The study foremost examined the social representations of the centrality of work, altruistic values, intrinsic values, extrinsic values, social values, leadership, and flexibility. Of the categories with associated factors measured, results showed for instance that development opportunities, social values and the variation of work were distinctive values among Generation Y graduates. Results also showed that balancing leisure with work was of great importance.

In this thesis, we investigate various problems in the representation theory of finite groups of Lie type. In Chapter 2, we hope to make sense of the last statement - we will introduce some background and notation that will be useful for the remainder of the thesis. In Chapter 3, we find bounds for the largest irreducible representation degree of a finite unitary group. In Chapter 4, we describe the block distribution and Brauer characters in cross characteristic for Sp₆(2ᵃ) in terms of the irreducible ordinary characters. This will be useful in Chapter 5 and Chapter 7, which focus primarily on the group Sp₆(2ᵃ) and contain the main results of this thesis, which we now summarize. Given a subgroup H ≤ G and a representation V for G, we obtain the restriction V|H of V to H by viewing V as an FH-module. However, even if V is an irreducible representation of G, the restriction V|H may (and usually does) fail to remain irreducible as a representation of H. In Chapter 5, we classify all pairs (V, H), where H is a proper subgroup of G = Sp₆(q) or Sp₄(q) with q even, and V is an l-modular representation of G for l ≠ 2 which is absolutely irreducible as a representation of H. This problem is motivated by the Aschbacher-Scott program on classifying maximal subgroups of finite classical groups. The local-global philosophy plays an important role in many areas of mathematics. In the representation theory of finite groups, the so-called "local-global" conjectures would relate the representation theory of G to that of certain proper subgroups, such as the normalizer of a Sylow subgroup. One might hope that these conjectures could be proven by showing that they are true for all simple groups. Though this turns out not quite to be the case, some of these conjectures have been reduced to showing that a finite set of stronger conditions hold for all finite simple groups. In Chapter 7, we show that Sp₆(q) and Sp₄(q), q even, are "good" for these reductions.