Between Coherent and Constructible Local Langlands Correspondences

TL;DR

This paper introduces a unifying framework using cyclic homology and circle actions to connect different geometric realizations of the local Langlands correspondence for both archimedean and non-archimedean fields.

Contribution

It proposes a general mechanism that interpolates between constructible and coherent sheaf approaches in local Langlands, linking various conjectures via derived algebraic geometry.

Findings

Relates coherent and constructible realizations of affine Hecke algebras.

Describes how circle actions connect smooth representations of $GL_n$.

Provides a method to derive the constructible correspondence from the coherent form.

Abstract

Refined forms of the local Langlands correspondence seek to relate representations of reductive groups over local fields with sheaves on stacks of Langlands parameters. But what kind of sheaves? Conjectures in the spirit of Kazhdan-Lusztig theory (due to Vogan and Soergel) describe representations of a group and its pure inner forms with fixed central character in terms of constructible sheaves. Conjectures in the spirit of geometric Langlands (due to Fargues, Zhu and Hellmann) describe representations with varying central character of a large family of groups associated to isocrystals in terms of coherent sheaves. The latter conjectures also take place on a larger parameter space, in which Frobenius (or complex conjugation) is allowed a unipotent part. In this article we propose a general mechanism that interpolates between these two settings. This mechanism derives from the theory of cyclic homology, as interpreted through circle actions in derived algebraic geometry. We apply this perspective to categorical forms of the local Langlands conjectures for both archimedean and non-archimedean local fields. In the nonarchimedean case, we describe how circle actions relate coherent and constructible realizations of affine Hecke algebras and of all smooth representations of $GL_n$, and propose a mechanism to relate the two settings in general. In the archimedean case, we explain how to use circle actions to derive the constructible local Langlands correspondence (in the form due to Adams-Barbasch-Vogan and Soergel) from a coherent form (a real counterpart to Fargues' conjecture): the tamely ramified geometric Langlands conjecture on the twistor line, which we survey.