Let r be a graph with vertex set V and for every v E V let Gv be a group with present ation (Sv I Rv). Let E ~ V X V be the set of pairs of adj acent vertices. Then we define the group G = Gr to be the group with presentation G = (SvVv E VI R; Vv E V, [Sv, SVI] = 1 iff (v,v') E E). In [2, LEMMA 3.3] it is shown that up to isomorphism G is independent of the choice of presentation of each group Gv. We call the group G a graph product of groups. Graph products include as special cases free products and direct products, corresponding to the graph G being dixcrete and complete respectively. If the vertex groups G; are infinite cyclic then G is called a graph group and we identify each vertex v with a fixed generator of the vertex group Gv• There is a normal form theorem for graph products which is a generalisation of the normal form theorem for free products and which was proved in [2]. In Part 1we give an alternative proof. We then move on to the study of automorphisms of graph products. In full generality this is an impossible task; however some progress can be made in certain special cases. We first consider the case where G is a graph group. Servatius in [1] gave a simple set of elements of Aut( G), which he calls elementary automorphisms, and proved that if certain conditions are imposed on the graph r then the elementary automorphisms generate Aut(G). In Part 2 we will prove that this holds for all finite graphs r. In Part 3 we study Aut( G) in the case where each vertex group Gv is cyclic of order p for some fixed prime p and we find a simple set of generators for Aut(G). In the case p = 2 we also obtain a presentation for Aut( G). In this case G is a right-angled Coxeter group

In the first half of this dissertation we study certain quotient algebras of preprojective algebras called no-cycle algebras N. These are studied via one-cycle algebras, which are introduced here. Results include detailed combinatorial information on N, and in certain special cases a presentation for N as a quiver with relations. In the second half we consider deformations of coordinate algebras of Kleinian singularities. Results include an explicit presentation for the deformations of a type D singularity. These two themes are tied together at the end by some mainly speculative comments about the role the various objects studied have to play in representation theory.

Dated 1983 / Bibliography: leaves 143-145 / 145 leaves ; 30 cm. / Title page, contents and abstract only. The complete thesis in print form is available from the University Library. / Thesis (Ph.D.)--University of Adelaide, 1984

Soit F une extension finie du corps des nombres p-adiques, de corps résiduel Fq. Pour un groupe réductif G sur F, les conjectures de Langlands prédisent une classification des représentations lisses irréductibles de G(F) en termes du groupe dual G^. En particulier, la donnée d’un homomorphisme de groupes duaux de H^ vers G^ doit se traduire par un transfert des représentations de H(F) vers G(F). Pour H = SO2n+1, et G = GL2n, l’injection canonique de H^ vers G^ fournit un transfert des représentations de H(F) vers G(F) qui a été obtenu récemment (pour les représentations génériques) par Jiang et Soudry.Cependant, leurs méthodes utilisent des arguments globaux et l’objet de ce travail consiste à décrire explicitement ce transfert, dans le cas particulier où n = 2 (le cas n = 1 étant déjà connu), et pour des représentations génériques de niveau zéro, lesquelles proviennent essentiellement de représentations du groupe réductif fini SO5 sur le corps résiduel de F. Pour cela, l’isomorphisme entre SO5 et PGSp4 et l’isogénie entre GL4 et GSO6 suggèrent que l’on peut réaliser un transfert entre les représentations de SO5 et celles de GL4 au moyen d’une correspondance de Howe. Nous présentons ici une généralisation des travaux de Srinivasan, qui nous permet d’obtenir la projection uniforme de la représentation de Weil associée à une paire duale de groupes de similitudes lorsque q est assez grand. / Let F be a p-adic field, and let k be its residue field. According to Langlands' conjectures, smooth irreducible representations of a reductive group G defined over F should be classified in terms of the dual groupe G^. In particular, given a homomorphism from H^ to G^, there should be a lift from the representations of H(F) to the representations of G(F). When H = SO2n+1 and G = GL2n, the canonical injection from H^ to G^ should induce a lift from representations of SO2n+1 to representations of GL2n, and this was studied by Jiang and Soudry.However, the arguments used by Jiang and Soudry are of global nature and the aim of this work is to describe explicitly this lift, when n = 2 (the case n = 1 is already known), for level zero generic representations, which are essentially determined by parameters over the finite residue field. Here the isomorphism between SO5 and PGSp4, as well as the isogeny between GL4 and GSO6 suggest that the lift could be realised by a sort of Howe correspondence.In this work, we generalize a result of Srinivasan and give the uniform projection of the Weil representation associated to a dual pair of similitude groups over Fq, when q is big enough.

On étudie la correspondance de Howe entre la catégorie de représentations complexes de G et celle de G’, pour des paires duales irréductibles (G,G’) définis sur des corps finis de caractéristique impaire. On établit la compatibilité entre la correspondance de Howe et les séries arbitraires de Harish-Chandra. On démontre comment obtenir des sous-représentations extrémales (i.e. minimales et maximales) de l’image d’une représentation irréductible unipotente de G. Finalement, on démontre comment l’étude de la correspondance de Howe entre séries d’Harish-Chandra arbitraires peut être ramenée à l’étude des séries unipotentes, et on utilise ceci pour étendre nos résultats sur les représentations extrémales aux représentations irréductibles arbitraires (i.e. pas forcément unipotentes) de G. / We study the Howe correspondence between the category of complex representations of G and that of G’, for irreducible dual pairs (G,G’) over finite fields of odd characteristic. We establish the compatibility between the Howe correspondence and arbitrary Harish-Chandra series. We define and prove the existence of extremal (i.e. minimal and maximal) irreducible sub-representations from the image of irreducible unipotent representations of G. Finally, we prove how the study of the Howe correspondence between arbitrary Harish-Chandra series can be brought to the study of unipotent series, and use this to extend our results on extremal representations to arbitrary (i.e. not necessarily unipotent) irreducible representations of G.