Deux applications arithmétiques des travaux d'Arthur

Abstract : We present two arithmetic applications of James Arthur's endoscopic classification of the discrete automorphic spectrum for symplectic and orthogonal groups. The first one consists in removing the irreducibility assumption in a theorem of Richard Taylor describing the image of complex conjugations by p-adic Galois representations associated with regular, algebraic, essentially self-dual, cuspidal automorphic representations of GL_{2n+1} over a totally real number field. We also extend it to the case of representations of GL_{2n} whose multiplicative character is ''odd''. We use a p-adic deformation argument, more precisely we prove that on the eigenvarieties for symplectic and even orthogonal groups, there are ''many'' points corresponding to (quasi-)irreducible Galois representations. Arthur's endoscopic classification is used to define these Galois representations, and also to transfer self-dual automorphic representations of the general linear group to these classical groups. The second application concerns the explicit computation of dimensions of spaces of automorphic or modular forms. Our main contribution is an algorithm computing orbital integrals at torsion elements of an unramified p-adic classical group, for the unit of the unramified Hecke algebra. It allows to compute the geometric side in Arthur's trace formula, and thus the Euler characteristic of the discrete spectrum in level one. Arthur's endoscopic classification allows to analyse precisely this Euler characteristic, and deduce the dimensions of spaces of level one automorphic forms. The dimensions of spaces of vector-valued Siegel modular forms, which constitute a more classical problem, are easily derived.
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Olivier Taïbi. Deux applications arithmétiques des travaux d'Arthur. Théorie des nombres [math.NT]. Ecole Polytechnique X, 2014. Français. ⟨pastel-01066463⟩

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