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Register a new userGalois $overline{mathbb F}_l$-monodromy, level fixing and Iharas lemma
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Authors
Pascal Boyer
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We exhibit cases of a level fixing phenomenon for galoisian automorphic representations of a CM field $F$, with dimension $d geq 2$. The proof rests on the freeness of the localized cohomology groups of KHT Shimura varieties and the strictness of its filtration induced by the spectral sequence associated to the filtration of stratification of the nearby cycles perverse sheaf at some fixed place $v$ of $F$. The main point is the observation that the action of the unipotent monodromy operator at $v$ is then given by those on the nearby cycles where its order of nilpotency modulo $l$ equals those in characteristic zero. Finally we infer some consequences concerning level raising and Iharas lemma.
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A key ingredient in the Taylor-Wiles proof of Fermat last theorem is the classical Iharas lemma which is used to rise the modularity property between some congruent galoisian representations. In their work on Sato-Tate, Clozel-Harris-Taylor proposed a generalization of the Iharas lemma in higher dimension for some similitude groups. The main aim of this paper is then to prove some new instances of this generalized Iharas lemma by considering some particular non pseudo Eisenstein maximal ideals of unramified Hecke algebras. As a consequence, we prove a level rising statement.
Persitence of non degeneracy is a phenomenon which appears in the theory of $overline{mathbb Q}_l$-representations of the linear group: every irreducible submodule of the restriction to the mirabolic subgroup of an non degenerate irreducible representation is non degenerate. This is no more true in general, if we look at the modulo $l$ reduction of some stable lattice. As in the Clozel-Harris-Taylor generalization of global Iharas lemma, we show that this property, called non degeneracy persitence, remains true for lattices given by the cohomology of Lubin-Tate spaces.
We study the torsion cohomology classes of Shimura varieties of type Kottwitz-Harris-Taylor and we show that up to an arbitrary place one can raise them to an automorphic representation. In application, to any mod $l$ system of Hecke eigenvalues appearing in the $bar{mathbb F}_l$-cohomology of a Shimuras variety of Kottwitz-Harris-Taylor type, we associate a $bar{mathbb F}_l$-Galois representation which Frobenius eigenvalues are given by Heckes. Compared to the highly more general construction of Scholze, we gain both the simplicity of the proof and the control at places ramified and at those dividing $l$.
By work of Belyi, the absolute Galois group $G_{mathbb{Q}}=mathrm{Gal}(overline{mathbb{Q}}/mathbb{Q})$ of the field $mathbb{Q}$ of rational numbers can be embedded into $A=mathrm{Aut}(widehat{F_2})$, the automorphism group of the free profinite group $widehat{F_2}$ on two generators. The image of $G_{mathbb{Q}}$ lies inside $widehat{GT}$, the Grothendieck-Teichmuller group. While it is known that every abelian representation of $G_{mathbb{Q}}$ can be extended to $widehat{GT}$, Lochak and Schneps put forward the challenge of constructing irreducible non-abelian representations of $widehat{GT}$. We do this virtually, namely by showing that a rich class of arithmetically defined representations of $G_{mathbb{Q}}$ can be extended to finite index subgroups of $widehat{GT}$. This is achieved, in fact, by extending these representations all the way to finite index subgroups of $A=mathrm{Aut}(widehat{F_2})$. We do this by developing a profinite version of the work of Grunewald and Lubotzky, which provided a rich collection of representations for the discrete group $mathrm{Aut}(F_d)$.
We prove that both local Galois representations and $(varphi,Gamma)$-modules can be recovered from prismatic F-crystals, from which we obtain a new proof of the equivalence of Galois representations and $(varphi,Gamma)$-modules.
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