Depth one part of Tannakian groups of filtrations

TL;DR

This paper describes the structure of the Lie algebra associated with a Tannakian group of a filtered object, focusing on the first graded piece and its relation to extensions, with applications to motives.

Contribution

It provides a new description of the first graded piece of the Lie algebra in terms of extensions without assuming semisimplicity or functoriality of the filtration.

Findings

The graded piece $Gr^F_{-1}u(M)$ is determined by certain extension classes.

A criterion for when $u(M)$ equals its trivial upper bound is established.

Applications to the structure of objects with fixed graded pieces and maximality conditions are presented.

Abstract

Let $(F_r M)_{r\in\mathbb{Z}}$ be a finite filtration on an object $M$ of a neutral Tannakian category $\mathbf{T}$ in characteristic zero. Let $u(M)=u^F(M)$ be the Lie algebra of the subgroup of the Tannakian fundamental group of $M$ that acts trivially on the associated graded $Gr^FM$. The filtration $F_\bullet M$ induces a filtration on the internal Hom $\underline{Hom}(M,M)$, which in turn induces a filtration $F_\bullet u(M)$ on $u(M)$. This filtration on $u(M)$ is concentrated in negative degrees. In this paper, we give a description of the graded piece $Gr^F_{-1}u( M)$ in terms of the extensions $F_{r+1}M/F_{r-1}M\in Ext^1(Gr^F_{r+1}M, Gr^F_{r}M)$. In particular, these extensions determine $Gr^F_{-1}u(M)$. Note that here we neither assume the filtration is functorial, nor we assume that $Gr^FM$ is semisimple. The problem of studying $u(M)$ in this generality is motivated by the desire to understand Tannakian groups associated to a mixed motive and its realizations, including realizations for which semisimplicity of realizations of pure motives is not known and realizations that lack an interesting functorial weight filtration. We also give two related applications. The first is an equivalent condition in the generality described above for when $u(M)$ coincides with its trivial upper bound $F_{-1}\underline{Hom}(M,M)$. This result generalizes earlier criteria for maximality of $u(M)$ obtained by various authors in special contexts or under limiting conditions. In the second application, we apply the results to the setting of a neutral Tannakian category with a functorial weight filtration $W_\bullet$. Combining the constructions of [arXiv:2307.15487] with our general maximality criterion we prove a result about the structure of the set of isomorphism classes of objects $M$ for which $Gr^WM$ is isomorphic to a given graded object $A$ and $u^W(M)=W_{-1}\underline{Hom}(M,M)$.