The formal theory of Tannaka duality

TL;DR

This paper develops a formal framework for Tannaka duality applicable over arbitrary commutative rings, extending classical results to broader algebraic contexts and applying it to p-adic Galois representations.

Contribution

It introduces a formal theory of Tannaka duality that generalizes existing results to non-field settings and connects to categories of filtered modules for p-adic Galois studies.

Findings

Provides sufficient conditions for categories to be equivalent to representation categories of affine groupoids or Hopf algebroids.

Develops a formal theory inspired by Street's monad theory.

Applies the theory to categories relevant to p-adic Galois representations.

Abstract

A Tannakian category is an abelian tensor category equipped with a fiber functor and additional structures which ensure that it is equivalent to the category of representations of some affine groupoid scheme acting on the spectrum of a field extension. If we are working over an arbitrary commutative ring rather than a field, the categories of representations cease to be abelian. We provide a list of sufficient conditions which ensure that an additive tensor category is equivalent to the category of representations of an affine groupoid scheme acting on an affine scheme, or, more generally, to the category of representations of a Hopf algebroid in a symmetric monoidal category. In order to do this we develop a "formal theory of Tannaka duality" inspired by Ross Street's "formal theory of monads." We apply our results to certain categories of filtered modules which are used to study p-adic Galois representations.