Irreducible representations of a finite group over a field are important because all representations of a group are direct sums of irreducible representations. Maschke tells us that if φ is a representation of the finite group G of order n on the m-dimensional space V over the field K of complex numbers and if U is an invariant subspace of φ, then U has a complementary reducing subspace W . The objective of this thesis is to find all irreducible representations of the dihedral group D2n. The reason we will work with the dihedral group is because it is one of the first and most intuitive non-abelian group we encounter in abstract algebra. I will compute the representations and characters of D2n and my thesis will be an explanation of these computations. When n = 2k + 1 we will show that there are k + 2 irreducible representations of D2n, but when n = 2k we will see that D2n has k + 3 irreducible rep- resentations. To achieve this we will first give some background in group, ring, module, and vector space theory that is used in representation theory. We will then explain what general representation theory is. Finally we will show how we arrived at our conclusion.

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This thesis concerns representation growth of finitely generated torsion-free nilpotent groups. This involves counting equivalence classes of irreducible representations and embedding this counting into a zeta function. We call this the representation zeta function. We use a new, constructive method to calculate the representation zeta functions of two families of groups, namely the Heisenberg group over rings of quadratic integers and the maximal class groups. The advantage of this method is that it is able to be used to calculate the p-local representation zeta function for all primes p. The other commonly used method, known as the Kirillov orbit method, is unable to be applied to these exceptional cases. Specifically, we calculate some exceptional p-local representation zeta functions of the maximal class groups for some well behaved exceptional primes. Also, we describe the Kirillov orbit method and use it to calculate various examples of p-local representation zeta functions for almost all primes p.

In this paper linear representations of finite groups are introduced, and the associated character theory with it. Some work of linear representations of the dihedral group $D_n$ and the symmetric group $S_n$ is presented. \\We also take a look at the finite matrix groups $\textbf{GL}(\mathbb{F}_q)$ and $\textbf{SL}(\mathbb{F}_q)$. The character table for $\textbf{SL}(\mathbb{F}_4)$ and its representation spaces in an implicit form are calculated. We define the standard representation $\varphi $ of $\textbf{SL}(\mathbb{F}_q)$ and prove that it is irreducible for an arbitrary finite field $\mathbb{F}_q$.

We provide a combinatorial interpretation of the frequency of any irreducible representation of Sn in representations of Sn arising from group actions on words. Recognizing that representations arising from group actions naturally split across orbits yields combinatorial interpretations of the irreducible decompositions of representations from similar group actions. The generalization from group actions on words to group actions on matrices gives rise to representations that prove to be much less transparent. We share the progress made thus far on the open problem of determining the irreducible decomposition of certain representations of Sm × Sn arising from group actions on matrices.

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Dans cette thèse, nous nous intéressons à des singularités particulières dans les variétés de caractères. Dans le premier chapitre, on justifie que les caractères de représentations irréductibles d'un groupe fuchsien vers un groupe de Lie complexe semi-simple forment une orbifolde. Le lieu orbifold (i.e. l'ensemble des points dont l'isotropie n'est pas triviale) est constitué des caractères de représentations exceptionnelles. Dans le second chapitre, nous décrivons précisément le lieu orbifold quand le groupe de Lie est le groupe projectif linéaire sur un espace vectoriel complexe dont la dimension est un nombre premier. Dans le troisième et le quatrième chapitre nous cherchons à classifier les groupes d'isotropies possibles à conjugaison près apparaissant quand le groupe de Lie est respectivement un quotient du groupe spécial linéaire pour un espace vectoriel complexe de dimension finie quelconque dans le troisième chapitre et un quotient du groupe de spin complexe dans le quatrième chapitre. / Ln this thesis, we want to understand some singularities in the character variety. ln a first chapter, we justify that the characters of irreducible representations from a Fuchsian group to a complex semi-simple Lie group is an orbifold. The orbifold locus is, then, the characters of bad representations. ln the second chapter, we focus on the case where the Lie group is the projectif linear group over a complex vector space whose dimension is a prime number. ln particular we give an explicit description of this locus. ln the third and fourth chapter, we describe the isotropy groups (i.e. the centralizers of bad subgroups) arising in the cases when the Lie group is a quotient of the special linear group of a complex vector space of finite dimension (third chapter) and when the Lie group is a quotient of a complex spin group in the fourth chapter.