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Register a new userLalg`ebre des invariants dun groupe de Coxeter agissant sur un mutiple de sa representation standard
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Authors
Loic Foissy
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Let G be a Coxeter group of type A_n, B_n, D_n or I_2(N), or a complex reflection group of type G(de,e,n). Let V be its standard representation and let k be an integer greater than 2. Then G acts on S(V)^{otimes k}. We show that the algebra of invariants (S(V)^{otimes k})^G is a free (S(V)^G)^{otimes k}-module of rank |G|^{k-1}, and that S(V)^{otimes k} is not a free (S(V)^{otimes k})^G-module.
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Let $V$ be an elementary abelian $2$-group and $X$ be a finite $V$-CW-complex. In this memoir we study two cochain complexes of modules over the mod2 Steenrod algebra $mathrm{A}$, equipped with an action of $mathrm{H}^{*}V$, the mod2 cohomology of $V$, both associated with $X$. The first, which we call the topological complex, is defined using the orbit filtration of $X$. The second, which we call the algebraic complex, is defined just in terms of the unstable $mathrm{A}$-module $mathrm{H}^*_V X$, the mod2 equivariant cohomology of $X$. Our study makes intensive use of the theory of unstable $mathrm{H}^{*}V$-$mathrm{A}$-modules which is a by-product of the researches on Sullivan conjecture. There is a noteworthy overlap between the topological part of our memoir and the paper Syzygies in equivariant cohomology in positive characteristic, by Allday, Franz and Puppe, which has just appeared; however our techniques are quite different from theirs (the name Steenrod does not show up in their article).
On the rank of Jacobians over function fields.} Let $f:mathcal{X}to C$ be a projective surface fibered over a curve and defined over a number field $k$. We give an interpretation of the rank of the Mordell-Weil group over $k(C)$ of the jacobian of the generic fibre (modulo the constant part) in terms of average of the traces of Frobenius on the fibers of $f$. The results also give a reinterpretation of the Tate conjecture for the surface $mathcal{X}$ and generalizes results of Nagao, Rosen-Silverman and Wazir.
Let X_d be the p-adic analytic space classifying the d-dimensional (semisimple) p-adic Galois representations of the absolute Galois group of Q_p. We show that the crystalline representations are Zarski-dense in many irreducible components of X_d, including the components made of residually irreducible representations. This extends to any dimension d previous results of Colmez and Kisin for d = 2. For this we construct an analogue of the infinite fern of Gouv^ea-Mazur in this context, based on a study of analytic families of trianguline (phi,Gamma)-modules over the Robba ring. We show in particular the existence of a universal family of (framed, regular) trianguline (phi,Gamma)-modules, as well as the density of the crystalline (phi,Gamma)-modules in this family. These results may be viewed as a local analogue of the theory of p-adic families of finite slope automorphic forms, they are new already in dimension 2. The technical heart of the paper is a collection of results about the Fontaine-Herr cohomology of families of trianguline (phi,Gamma)-modules.
Consider the ring of holomorphic function germs in $C^n$ and denote by $M$ the maximal ideal of this ring. For any a holomorphic function germ $f$ with an isolated critical point, the finite determinacy theorem (Mather-Tougeron) asserts that there exists some $k$, such that $f+g$ can be brought back to $f$, via a holomorphic change of variables, for any $g in M^k$. In this paper, a generalisation of this theorem for functions defined in a neighbourhood of a Stein compact subset and for an arbitrary ideal is given.
Let F be a finite extension of Qp, O_F its ring of integers and E a finite extension of Fp. The natural action of the unit group O_F* on O_F extends in a continuous action on the Iwasawa algebra E[[O_F]]. In this work, we show that non zero ideals of E[[O_F]] which are stable under O_F* are open. As a consequence, we deduce the fidelity of the action of E[[U]], with U the subgroup of upper unipotent matrices in GL2(O_F) on an irreducible admissible smooth E-representation of GL2(F). ----- Soit F une extension finie de Qp, danneau des entiers O_F et E une extension finie de Fp. Laction naturelle du groupes des unites O_F* sur O_F se prolonge alors en une action continue sur lalg`ebre dIwasawa E[[O_F]]. Dans ce travail, on demontre que les ideaux non nuls de E[[O_F]] stables par O_F* sont ouverts. En particulier, on en deduit la fidelite de laction de lalg`ebre dIwasawa des matrices unipotentes superieures de GL2(O_F) sur une representation lisse irreductible admissible de GL2(F).
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