Global Parabolic Induction and Abstract Automorphicity

TL;DR

This paper systematically studies a category of abstract automorphic representations for GL(2) over function fields, revealing structural decompositions and new perspectives on intertwining operators and L-functions.

Contribution

It proves structural theorems about the category, including its decomposition into cuspidal and Eisenstein parts, and offers a new viewpoint on intertwining operators as self-duality of induction.

Findings

Category admits an automorphic parabolic restriction and induction functors.

Decomposition into cuspidal and Eisenstein components analogous to Bernstein decomposition.

Intertwining operator viewed as self-duality of induction functor.

Abstract

In arXiv:2011.03313, the author has constructed a category of abstractly automorphic representations for $\mathrm{GL}(2)$ over a function field $F$. This is a symmetric monoidal Abelian category, constructed with the goal of having the irreducible automorphic representations as its simple objects. The goal of this paper is to systematically study this category. We will prove several structural theorems about this category. We will show that it admits an adjoint pair $(r^\mathrm{aut},i^\mathrm{aut})$ of automorphic parabolic restriction and induction functors, respectively. This will allow us to show that the category of abstractly automorphic representations decomposes into cuspidal and Eisenstein components, in analogy with the Bernstein decomposition of the category of $p$-adic representations. Moreover, along the way, we will give a new perspective on the intertwining operator of $\mathrm{GL}(2)$ (and on the functional equation for Eisenstein series), as a form of self-duality of the functor of parabolic induction. We will also illustrate how the role of analytic continuation in this theory can be thought of as trivializing a twist by a certain line bundle, which corresponds to an L-function via the results of arXiv:2012.03068. If one chooses to keep the twist as a part of the theory, then one avoids the need for analytic continuation.