Etale and crystalline companions, II

TL;DR

This paper proves the existence of crystalline companions for certain $ ext{l}$-adic sheaves over finite fields, confirming a conjecture of Deligne and linking $ ext{l}$-adic and $p$-adic coefficients through advanced Langlands and Drinfeld techniques.

Contribution

It establishes the existence of crystalline companions for algebraic $ ext{l}$-adic Weil sheaves on smooth schemes over finite fields, confirming Deligne's conjecture and connecting $ ext{l}$-adic and $p$-adic theories.

Findings

Existence of crystalline companions for algebraic $ ext{l}$-adic sheaves.

Transfer of properties between crystalline and étale coefficient objects.

Proof of Wan's theorem on $p$-adic meromorphicity of unit-root $L$-functions.

Abstract

Let $X$ be a smooth scheme over a finite field of characteristic $p$. In answer to a conjecture of Deligne, we establish that for any prime $\ell \neq p$, an $\ell$-adic Weil sheaf on $X$ which is algebraic (or irreducible with finite determinant) admits a crystalline companion in the category of overconvergent $F$-isocrystals, for which the Frobenius characteristic polynomials agree at all closed points (with respect to some fixed identification of the algebraic closures of $\mathbb{Q}$ within fixed algebraic closures of $\mathbb{Q}_\ell$ and $\mathbb{Q}_p$). The argument depends heavily on the free passage between $\ell$-adic and $p$-adic coefficients for curves provided by the Langlands correspondence for $\mathrm{GL}_n$ over global function fields (work of L. Lafforgue and T. Abe), and on the construction of Drinfeld (plus adaptations by Abe-Esnault and Kedlaya) giving rise to \'etale companions of overconvergent $F$-isocrystals. As corollaries, we transfer a number of statements from crystalline to \'etale coefficient objects, including properties of the Newton polygon stratification (results of Grothendieck-Katz and de Jong-Oort-Yang) and Wan's theorem (previously Dwork's conjecture) on $p$-adic meromorphicity of unit-root $L$-functions.