<p dir="ltr">A group is said to be matricially (Frobenius) stable if every function from the group to unitary matrices that is "almost multiplicative" in the point operator (Frobenius) norm topology is "close" to a genuine unitary representation in the same topology. A result of Dadarlat shows that for a large class of groups, non-torsion even cohomology obstructs matricial stability. However, the proof doesn't generate explicit almost multiplicative maps that are far from genuine representations. In this paper, we compute explicit almost homomorphisms for all finitely generated groups with a non-torsion 2-cohomology class with a residually finite central extension. We use similar techniques to show that finitely generated nilpotent groups are Frobenius stable if and only if they are virtually cyclic, and that a finitely generated group with a non-torsion 2-cohomology class that can be written as a cup product of two 1-cohomology classes is not Frobenius stable.</p><p><br></p>

We use the theory of dual of Fréchet-Schwartz (DFS) spaces to establish a sufficient condition for top-degree solvability for the differential complex associated to a hypocomplex locally integrable structure. As an application, we show that the top-degree cohomology of left-invariant hypocomplex structures on a compact Lie group can be computed only by using left-invariant forms, thus reducing the computation to a purely algebraic one. In the case of left-invariant elliptic involutive structures on compact Lie groups, under certain reasonable conditions, we prove that the cohomology associated to the involutive structure can be computed only by using left-invariant forms. / Usamos a teoria da espaços duais de Fréchet-Schwartz (DFS) para estabelecer uma condição suficiente para resolubilidade em grau máximo para o complexo associado a estrutuas localmente integráveis hipocomplexas. Como aplicação, provamos que a cohomologia de estruturas hipocomplexas invariantes à esquerda podem ser calculadas usando apenas formas invariantes à esquerda, assim reduzindo o cálculo a um método puramente algébrico. No caso de estruturas invariantes à esquerda, sob certas condições razoáveis, provamos que a cohomologia associada à estrutura pode ser calculada usando apenas formas invariantes à esquerda.

Dans cette thèse, nous nous s’intéressons au calcul des groupes de cohomologie de Dolbeault feuilletée H0∗L (M) de certaines laminations complexes. Ceci revient à résoudre le problème du ∂ le long des feuilles ∂Lα = ω. (Ici M est un espace métrique ou une variété dans le cas où L est un feuilletage F.) Trois situations ont été étudiées de manière explicite.1. Soit M = Ω un ouvert de C × R muni du feuilletage F dont les feuilles sont les sections Ωt = {z ∈ C : (z, t) ∈ Ω} ; on dira que F est le feuilletage canonique de Ω. Sous certaines conditions sur Ω et de croissance sur la forme feuilletée ω, nous montrons que l’´équation ∂Fα = ω a une solution.2. On se donne une suite (αn)n≥1 strictement croissante avec α1 = −1 et convergeant vers 1. Dans C × R on considère les points A = (0, 1) et An = (0, αn) pour n ≥ 1. Pour tout n ≥ 1, soient Sn la sphère de C × R de diamètre le segment [AnA] et E la réunion de toutes ces sphères. Alors E est un sous-espace métrique compact et connexe de C × R. Soit γ : E −→ E l’homéomorphisme défini par γ(w,u) = (ρn(w),u) lorsque (w, u) ∈ Sn où ρn est la rotation dans C d’angle 2πn. La suspension de γ donne une lamination complexe L dont les feuilles sont des surfaces de Riemann toutes équivalentes à C*. Pour cet exemple, nous montrons que l’espace vectoriel H01(L) est nul.3. On considère la variété M = C × Rn \ {(0, 0)} (les coordonnées d’un point seront notées (z,t)) qu’on munit du feuilletage complexe F défini par le système différentiel dt1 = • • • = dn = 0. Le difféomorphisme γ : (z, t) ∈ Mf7−→ (λz, λt) ∈ M (avec 0 < λ < 1) agit sur M de façon libre et propre ; en plus, c’est un automorphisme de F ; F induit alors sur le quotient M = M/γ (qui est difféomorphe `à Sn+1 × S1) un feuilletage complexe F par surfaces de Riemann. Nous montrons que les espaces vectoriels de cohomologie de Dolbeault feuilletée H00 F (M) et H01F (M) sont isomorphes à C. / In this thesis, we are interested in computing the foliated Dolbeault cohomology groups H0∗L (M) for some complex laminations. This amounts to solving the problem of the ∂ along the leaves ∂Lα = ω. (Here M is a metric space or a differentiable manifold if L is a foliation F.) Three situations were considered explicitly.1. Let M = Ω be an open set of C×R equipped with the foliation F whose leaves are the sections Ωt = {z ∈ C(z, t) ∈ Ω}; we say that F is the canonical foliation of Ω. Under certain conditions on Ω and growth conditions on the foliated form ω, we show that the equation ∂Fα = ω has a solution.2. Let (αn)n≥1 be a sequence of real numbers, strictly increasing with α1 = −1 and converging to 1. In C × R we consider the points A = (0, 1) and An = (0, αn) for n ≥ 1. For all n ≥ 1, let Sn be the sphere of C × R with a diameter segment [AnA] and E the union of all these spheres. Then E is a compact and connected subset of C × R. Let γ : E −→ E the homeomorphism defined by γ(w,u) = (ρn(w),u), where (w,u) ∈ Sn and ρn is the rotation in C with angle 2πn. The suspension of γ gives rise to a complex lamination L whose leaves are all equivalent Riemann surfaces isomorphic to C∗. For This example we show that the vector space H01 (L) is zero.3. Consider the manifold M = C × Rn \ {(0, 0)} (the coordinates of a point are denoted (z,t)) endowed with the complex foliation F defined by the differential system dt1 = • • • = dn = 0. The diffeomorphism γ : (z, t) ∈ M −→ (λz, λt) ∈ M (where 0 < λ < 1) acts on M freely and properly ; moreover it is an automorphism of the complex foliation F ; then F induces on the quotient M = M/γ (which is diffeomorphic to S n+1 × S1) a complex foliation F by Riemann surfaces. All leaves are isomorphic to C except one of them which is an elliptic curve. We show that the vector spaces H00 F (M) and H01F (M) of foliated Dolbeault cohomology are isomorphic to C.

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In this dissertation, we developed a mathematical framework for cohomological field theories (CohFTs) in the language of ``QK-manifolds', which unifies the previous ones in (Baulieu and Singer 1988; Baulieu and Singer 1989; Ouvry, Stora, and Van Baal 1989; Atiyah and Jeffrey 1990; Birmingham et al. 1991; Kalkman 1993; Blau 1993). Within this new framework, we classified the (gauge invariant) solutions to the descent equations in CohFTs (with gauge symmetries). We revisited Witten’s idea of topological twisting and showed that the twisted super-Poincaré algebra gives rise naturally to a ``QK-structure'. We also generalized the Mathai-Quillen construction of the universal Thom class via a variational bicomplex lift of the equivariant cohomology. Our framework enables a uniform treatment of examples like topological quantum mechanics, topological sigma model, and topological Yang-Mills theory.

<p dir="ltr">In this thesis, we prove the rationality results for the ratio of the critical values of certain <i>L</i>-functions, which appear in the constant term of Eisenstein series associated with the exceptional group <i>G</i>_<em>2</em> over a totally imaginary field. Our methodology builds upon the works of Harder and Raghuram, who established rationality results for special values of Rankin-Selberg <i>L</i>-functions for<i> </i><i>GL</i>_<em>n</em><i>× GL</i>_<em>n'</em> by studying the rank-one Eisenstein cohomology of the ambient group <i>GL</i>_n+n' over a totally real field, as well as its generalization by Raghuram [35] for the case over a totally imaginary field.</p><p dir="ltr">The <i>L</i>-functions in this thesis were constructed using the Langlands-Shahidi method for <i>G</i>_<em>2</em> over a totally imaginary field, attached to maximal parabolic subgroups. This is the first instance of applying the Harder-Raghuram method to an exceptional group, and the first case involving more than one function appearing in the constant term. Our results demonstrate the relationship between the rationality of different <i>L</i>-functions appearing in the constant term, allowing one to prove the rationality of one <i>L</i>-function based on the known rationality result of another <i>L</i>-functions.</p>

Neste trabalho inicialmente estudamos o rank da co-homologia do espaço dos pontos fixos de uma \'Z IND.p\' - ação semilivre sobre espaços X~p \' S POT. n\' x \'S POT.n\' e X~p \'S POT.n\' x \'S POT.n\' x \'S POT.n\' , com n>0. Em seguida, estudamos uma extensão para ações de p-grupos sobre espaços X~p \'S POT.n\' X \'S POT.m\', com 0< n \'< OU =\' m. Como parte do material utilizado demos uma descrição do diferencial d1 de uma seqüência espectral que converge para co-homologia equivariante de Tate, bem como uma versão da Fórmula de Künneth para a co-homologia equivariante de Tate. Na parte final, motivado pelo problemas de descrição de espaços de órbita de ações de grupos infinito, calculamos as co-homologias dos grupos virtualmente cíclicos (\'Z IND.a\' X| \' Z IND. b\' )X| Z e [\'Z POT.a\' X|(\'Z IND.b\' X \'Q IND. 2 POT.i\') ]X| Z / In this work is studied the rank of the fixed point set of a semifree action on spaces X~p \'S POT.n\' X \'S POT.n\' and X~p \'S POT.n\' X \'S POT.n\' X \'S POT.n\' , with n>0. We also consider the extension of the result for actions of p-groups on spaces X~p \'SPOT.n\' X \' S POT.m\' , with 0<n \'< OR =\' m. As result of the techniques used, we give a description of the differential d1 of a spectral sequence that converges to Tate equivariant cohomology, as well a version of the Künneth Formule to Tate equivariant cohomology. At the end, motivated by the space form problem for infinite groups we compute the cohomology of the virtually cyclic groups (\'Z IND. a\' X| \'Z IND. b\' )X| Z and [\'Z IND.a\' X|(\'Z IND. b\' X \'Q IND2 POT. i\' )] X| Z

Dans cette thèse nous étudions la filtration par le poids sur la cohomologie de De Rham d’une courbe hyperelliptique C définie sur une extension finie de Qp et à réduction semi-stable. L’objectif est de fournir des algorithmes calculant explicitement, étant donné une équation de C, les bases des crans de la filtration par le poids ainsi que la matrice de l’accouplement de Poincaré. Dans le premier chapitre, nous mettons en place des outils relatifs à la cohomologie de De Rham algébrique de la courbe hyperelliptique. Nous construisons une base adaptée de la cohomologie de De Rham de C, nous établissons une formule explicite pour le cup-produit et la trace, et enfin nous proposons un algorithme calculant la matrice de l’accouplement de Poincaré. Le deuxième chapitre est consacré à la description explicite de la flèche induite par l’inclusion du tube d’un point double sur les espaces de cohomologie. C’est l’ingrédient essentiel pour pouvoir décrire la filtration par le poids sur la cohomologie de De Rham de C. À cette fin nous nous plaçons dans le cadre de la géométrie analytique à la Berkovich et nous introduisons puis développons les notions de point résiduellement singulier standard et de forme apparente de l’équation de la courbe. Dans le troisième et dernier chapitre, nous faisons la synthèse des résultats obtenus et achevons la description de la filtration par le poids. Enfin, nous donnons les algorithmes calculant les bases de Fil0 et Fil1. Pour les algorithmes obtenus dans la thèse nous proposons une implémentation en sage, ainsi que des exemples concrets sur des courbes de genre un et deux. / In this thesis we study the weight filtration on the De Rham cohomology of an hyperelliptic curve C defined over a finite extension of Qp and with semi-stable reduction. The goal is to provide algorithms computing explicitly, given an equation of C, the basis of the weight filtration’s spaces as well as the matrix of the Poincaré pairing. In the first chapter we introduce tools related to the algebraic De Rham cohomology of the hyperelliptic curve. We build a suitable basis of the De Rham cohomology of C, we establish explicit formulae for the cup-product and the trace, and we give an algorithm computing the matrix of the Poincaré pairing. The second chapter is dedicated to the explicit description of the morphism induced by the inclusion of the tube of a double point on the cohomology spaces. It is the main ingredient that allows us to describe the weight filtration on the De Rham cohomology of C. To achieve that, we use the framework of the Berkovitch analytical geometry. We introduce and then we develop the notion of standard residually singular points and the notion of apparent form of the curve’s equation. In the third and last chapter, we synthesize all the results and we complete the description of the weight filtration. Finally, we give the algorithms that compute the basis of Fil0 and Fil1. For each of our algorithm, we propose a sage implementation and concrete examples on genus one and two curves.

In 1976, Springer defined a correspondence making a link between the irreducible ordinary (characteristic zero) representations of a Weyl group and the geometry of the associated nilpotent variety. In this thesis, we define a modular Springer correspondence (in positive characteristic), and we show that the decomposition numbers of a Weyl group (for example the symmetric group) are particular cases of decomposition numbers for equivariant perverse sheaves on the nilpotent variety. We calculate explicitly the decomposition numbers associated to the regular and subregular classes, and to the minimal and trivial classes. We determine the correspondence explicitly in the case of the symmetric group, and show that James's row and column removal rule is a consequence of a smooth equivalence of nilpotent singularities obtained by Kraft and Procesi. The first chapter contains generalities about perverse sheaves with Z_l and F_l coefficients.

This paper is an overview of the developments and generalizations of Tate Cohomology. The number of such generalizations is high and the literature on many of them is vast. Hence, we do not pretend to give a complete account of all the branches that have developed from the original ideas of Tate. This is rather an overview of how the ideas developed. / Este artículo es una revisión del desarrollo y generalizaciones de la cohomología de Tate. El número de tales generalizaciones es alto y la literatura en torno a muchas de ellas es vasta. Por consiguiente, no pretendemos dar un recuento completo de las ramas que se desprenden de las ideas originales de Tate; esto más bien representa un bosquejo de cómo estas ideas se han ido desarrollando.