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).

In this thesis we propose a description of the operad defining post-Lie algebras in terms of rooted trees and we discuss some applications of such a construction. In particular, we re-derive both the free post-Lie algebra defined in [22] and the main result of the paper [8]. Furthermore, a possible extension of the concept of symmetric brace algebra to the category of the post-Lie algebras is proposed. / Nessa tese propomos a descrição da operad que define as álgebras pós-Lie em termos de árvores enraizadas e discutimos algumas aplicações dessa construção. Em particular, nós obtemos novamente a álgebra pós-Lie livre definida em [22] e o resultado principal do artigo [8]. Além disso, uma possível extensão do conceito de álgebra brace simétrica à categoria de álgebras pós-Lie é apresentada.

As Álgebras Quânticas foram recentemente introduzidas como uma generalização das álgebras de Lie clássicas e estão sendo intensamente investigadas, tanto de um ponto de vista matemático quanto em aplicações envolvendo problemas de Mecânica Estatística e Física Molecular. As representações dessas álgebras podem ser construídas a partir de técnicas tradicionais e apresentam novidades se o parâmetro de deformação q for uma raiz complexa da unidade, e neste caso pode ocorrer perda de irredutibilidade e conseqüentemente alterações nas dimensões dessas representações. Primeiramente, estudamos as representações no caso clássico, a seguir introduzimos as deformações quânticas nas relações de comutação envolvendo os geradores associados as raízes simples. Posteriormente, estudamos especificamente o caso em que q é uma raiz complexa da unidade, à procura de novas reduções dimensionais que não aparecem no caso clássico. Mais precisamente, nos detemos ao estudo das representações das álgebras deformadas Uq(su(2)) e Uq(sl(3)), determinando suas a dimensões, os vetores de base do espaço portador e as suas matrizes irredutíveis. Por fim, calculamos o operador de Casimir quadrático deformado procurando saber como ficam as regras de ramificação da cadeia Uq(sl(3)) &#8835 Uq(sl(2)). / The Quantum Algebras has been recently introduced as a generalization of classical Lie algebras. The representations of these algebras can be built from the traditional techniques and arise novelty, if the parameter of deformation q is a root of unity, in this case, can occur loss of irreducibility and consequently alteration in the dimension of these representations. First of all, we study the representations in the classic case, after that we introduce the quantum deformation in the commuting relations involving the generators associated with the simple roots. Subsequently we studied specifically the case that q is a root of unity, searching for dimensional reduction that do not appear in the classic algebras. More exactly, we studied the deformed representations of Uq(su(2)) e Uq(sl(3)), determining t heir dimensions, t he base vectors of t he carrier space and their irreducible matrices. Finally, we calculated the deformed quadratic Casimir operator in the chain Uq(sl(3)) &#8835 Uq(sl(2)).

Following the work of Conant and Vogtmann on determining the homology of the group of outer automorphisms of a free group, a new nontrivial class in the rational homology of Outer space is established for the free group of rank eight. The methods started in [8] are heavily exploited and used to create a new graph complex called the space of good chord diagrams. This complex carries with it significant computational advantages in determining possible nontrivial homology classes.Next, we create a basepointed version of the Lie operad and explore some of it proper- ties. In particular, we prove a Kontsevich-type theorem that relates the Lie homology of a particular space to the cohomology of the group of automorphisms of the free group.

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).

This thesis is about extended affine Lie algebras and extended affine Weyl groups. In Chapter I, we provide the basic knowledge necessary for the study of extended affine Lie algebras and related objects. In Chapter II, we show that the well-known twisting phenomena which appears in the realization of the twisted affine Lie algebras can be extended to the class of extended affine Lie algebras, in the sense that some extended affine Lie algebras (in particular nonsimply laced extended affine Lie algebras) can be realized as fixed point subalgebras of some other extended affine Lie algebras (in particular simply laced extended affine Lie algebras) relative to some finite order automorphism. We show that extended affine Lie algebras of type A₁, B, C and BC can be realized as twisted subalgebras of types A_§¤(l ¡Ã 2) and D algebras. Also we show that extended affine Lie algebras of type BC can be realized as twisted subalgebras of type C algebras. In Chapter III, the last chapter, we study the Weyl groups of reduced extended affine root systems. We start by describing the extended affine Weyl group as a semidirect product of a finite Weyl group and a Heisenberg-like normal subgroup. This provides a unique expression for the Weyl group elements which in turn leads to a presentation of the Weyl group, called a presentation by conjugation. Using a new notion, called the index, which is an invariant of the extended affine root systems, we show that one of the important features of finite and affine root systems (related to Weyl group) holds for the class of extended affine root systems. We also show that extended affine Weyl groups (of index zero) are homomorphic images of some indefinite Weyl groups where the homomorphism and its kernel are given explicitly.

Diss.--Mathematik--Chemnitz--Technische Universität, 1998. / Bibliogr. p. 103-105.

No description available.

Darmstadt, Techn. University, Diss., 2007.

Techn. Univ., Habil.-Schr.--Clausthal, 2000. / Literaturverz. S. [256] - 262.