If G is a nite group and if X is a G-space, then a Bredon RO(G)-graded equivariantcohomology theory is dened on X. Furthermore, if X is a G-manifold, thereexists a natural Čech hypercohomology theory on X. While Bredon RO(G)-gradedcohomology is important in the theoretical aspects, the Čech cohomology is indispensablewhen computing the cohomology groups. The purpose of this dissertation is toconstruct an isomorphism between these two types of cohomology theories so that theinterplay becomes deeper between the theory and concretely computing cohomologygroups of classical objects. Also, with the aid of Čech cohomology, we can naturallyextend the Bredon cohomology to the more generalized Deligne cohomology.In order to construct such isomorphism, on one hand, we give a new constructionof Bredon RO(G)-graded equivariant cohomology theory from the sheaf-theoreticviewpoint. On the other hand, with Illman's theorem of smooth G-triangulation ofa G-manifold, we extend the existence of good covers from the nonequivariant tothe equivariant case. It follows that, associated to an equivariant good cover of aG-manifold X, there is a bounded spectral sequence converging to Čech hypercohomologywhose E1 page is isomorphic to the E1 page of a Segal spectral sequence whichconverges to the Bredon RO(G)-graded equivariant cohomology. Furthermore, Thisisomorphism is compatible with the structure maps in the two spectral sequences. So there is an induced isomorphism between two limiting objects, which are exactly theČech hypercohomology and the Bredon RO(G)-graded equivariant cohomology.We also apply the above results to real varieties and obtain a quasi-isomorphismbetween two commonly used complexes of presheaves.

If G is a nite group and if X is a G-space, then a Bredon RO(G)-graded equivariantcohomology theory is dened on X. Furthermore, if X is a G-manifold, thereexists a natural Čech hypercohomology theory on X. While Bredon RO(G)-gradedcohomology is important in the theoretical aspects, the Čech cohomology is indispensablewhen computing the cohomology groups. The purpose of this dissertation is toconstruct an isomorphism between these two types of cohomology theories so that theinterplay becomes deeper between the theory and concretely computing cohomologygroups of classical objects. Also, with the aid of Čech cohomology, we can naturallyextend the Bredon cohomology to the more generalized Deligne cohomology.In order to construct such isomorphism, on one hand, we give a new constructionof Bredon RO(G)-graded equivariant cohomology theory from the sheaf-theoreticviewpoint. On the other hand, with Illman's theorem of smooth G-triangulation ofa G-manifold, we extend the existence of good covers from the nonequivariant tothe equivariant case. It follows that, associated to an equivariant good cover of aG-manifold X, there is a bounded spectral sequence converging to Čech hypercohomologywhose E1 page is isomorphic to the E1 page of a Segal spectral sequence whichconverges to the Bredon RO(G)-graded equivariant cohomology. Furthermore, Thisisomorphism is compatible with the structure maps in the two spectral sequences. So there is an induced isomorphism between two limiting objects, which are exactly theČech hypercohomology and the Bredon RO(G)-graded equivariant cohomology.We also apply the above results to real varieties and obtain a quasi-isomorphismbetween two commonly used complexes of presheaves.

Este trabalho estuda a demonstração de existência de uma teoria de cohomologia em estruturas o-minimais arbitrárias, conforme o trabalho de Edmundo, Jones e Peatfield. / This work studies the proof of the existence of sheaf cohomology theory in arbitrary o-minimal structures, following the work of Edmundo, Jones and Peatfield.

Este trabalho estuda a demonstração de existência de uma teoria de cohomologia em estruturas o-minimais arbitrárias, conforme o trabalho de Edmundo, Jones e Peatfield. / This work studies the proof of the existence of sheaf cohomology theory in arbitrary o-minimal structures, following the work of Edmundo, Jones and Peatfield.

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We study a quantum mechanical $\sigma$-model whose target space is a hyperKähler cone. As shown by Singleton, [184], such a theory has superconformal invariance under the algebra $\mathfrak{osp}(4^*|4)$. One can formally define a superconformal index that counts the short representations of the algebra. When the hyperKähler cone has a projective symplectic resolution, we define a regularised superconformal index. The index is defined as the equivariant Hirzebruch index of the Dolbeault cohomology of the resolution, hereafter referred to as the index. In many cases, the index can be explicitly calculated via localisation theorems. By limiting to zero the fugacities in the index corresponding to an isometry, one forms the index of the submanifold of the target space invariant under that isometry. There is a limit of the fugacities that gives the Hilbert series of the target space, and often there is another limit of the parameters that produces the Poincaré polynomial for $\mathbb C^\times$-equivariant Borel-Moore homology of the space. A natural class of hyperKähler cones are Nakajima quiver varieties. We compute the index of the $A$-type quiver varieties by making use of the fact that they are submanifolds of instanton moduli space invariant under an isometry. Every Nakajima quiver variety arises as the Higgs branch of a three dimensional $\mathcal N =4$ quiver gauge theory, or equivalently the Coulomb branch of the mirror dual theory. We show the equivalence between the descriptions of the Hilbert series of a line bundle on the ADHM quiver variety via localisation, and via Hanany's monopole formula. Finally, we study the action of the Poisson algebra of the coordinate ring on the Hilbert series of line bundles. We restrict to the case of looking at the Coulomb branch of balanced $ADE$-type quivers in a certain infinite rank limit. In this limit, the Poisson algebra is a semiclassical limit of the Yangian of $ADE$-type. The space of global sections of the line bundle is a graded representation of the Poisson algebra. We find that, as a representation, it is a tensor product of the space of holomorphic functions with a finite dimensional representation. This finite dimensional representation is a tensor product of two irreducible representations of the Yangian, defined by the choice of line bundle. We find a striking duality between the characters of these finite dimensional representations and the generating function for Poincaré polynomials.

Neste trabalho estenderemos os resultados obtidos por Hara [34] e J. Jaworowski [38] substituindo as G-variedades por G-variedades generalizadas sobre Z. Além disso, provamos uma fórmula de comparação geral para grau de aplicações de uma variedade generalizada sobre uma esfera que são equivariantes com respeito a ações de grupos finitos, obtendo uma generalização do resultado de A. Kushkuley e Z. Balanov [40] / In this work, we extend the results obtained by Y. Hara [34] and J. Jaworowski [38] by replacing the free G-manifolds by free generalized G-manifolds over Z. Moreover, we prove a general comparison formula for degrees of equivariant maps from a generalized manifold to a sphere which are equivariant with respect to finite group actions, obtaining a generalization of the result of A. Kushkuley and Z. Balanov [40]

Neste trabalho estenderemos os resultados obtidos por Hara [34] e J. Jaworowski [38] substituindo as G-variedades por G-variedades generalizadas sobre Z. Além disso, provamos uma fórmula de comparação geral para grau de aplicações de uma variedade generalizada sobre uma esfera que são equivariantes com respeito a ações de grupos finitos, obtendo uma generalização do resultado de A. Kushkuley e Z. Balanov [40] / In this work, we extend the results obtained by Y. Hara [34] and J. Jaworowski [38] by replacing the free G-manifolds by free generalized G-manifolds over Z. Moreover, we prove a general comparison formula for degrees of equivariant maps from a generalized manifold to a sphere which are equivariant with respect to finite group actions, obtaining a generalization of the result of A. Kushkuley and Z. Balanov [40]

Mémoire numérisé par la Division de la gestion de documents et des archives de l'Université de Montréal.

Soit G un groupe algébrique semi-simple sur un corps de caractéristique 0. Ce mémoire discute d'un théorème d'annulation de la cohomologie supérieure du faisceau D des opérateurs différentiels sur une variété de drapeaux de G. On démontre que si P est un sous-groupe parabolique de G, alors H^i(G/P,D)=0 pour tout i>0. On donne en fait trois preuves indépendantes de ce théorème. La première preuve est de Hesselink et n'est valide que dans le cas où le sous-groupe parabolique est un sous-groupe de Borel. Elle utilise un argument de suites spectrales et le théorème de Borel-Weil-Bott. La seconde preuve est de Kempf et n'est valide que dans le cas où le radical unipotent de P agit trivialement sur son algèbre de Lie. Elle n'utilise que le théorème de Borel-Weil-Bott. Enfin, la troisième preuve est attribuée à Elkik. Elle est valide pour tout sous-groupe parabolique mais utilise le théorème de Grauert-Riemenschneider. On présente aussi une construction détaillée du faisceau des opérateurs différentiels sur une variété. / Let G be a semisimple algebraic group on a field of characteristic 0. This thesis discusses a vanishing theorem for the higher cohomology of the sheaf D of differential operators on a flag variety of G. We show that if P is a parabolic subgroup of G, then H^i(G/P,D)=0 for all i>0. In fact, we give three independent proofs of this theorem. The first proof, due to Hesselink, only works if the parabolic subgroup P is a Borel subgroup. It uses a spectral sequence argument as well as the Borel-Weil-Bott theorem. The second proof, due to Kempf, only works if the unipotent radical of P acts trivially on its Lie algebra. It only uses the Borel-Weil-Bott theorem. Finally, the third proof, due to Elkik, is valid for any parabolic subgroup. However, it uses the Grauert-Riemenschneider theorem. We also present a detailled construction of the sheaf of differential operators on a variety.