Remarks on motives of abelian type

TL;DR

This paper explores the properties and criteria for motives of abelian type, including their behavior under field extensions, relations to Murre's conjectures, and Chow group characterizations, advancing understanding in algebraic geometry.

Contribution

It establishes new criteria for motives of abelian type, links Murre's conjectures to products of curves over algebraically closed fields, and analyzes their behavior under field extensions.

Findings

Motives becoming of abelian type after base extension are of abelian type.

Finite-dimensionality of motives is preserved under field extension.

Chow groups characterize motives of abelian type via algebraically trivial cycles.

Abstract

A motive over a field $k$ is of abelian type if it belongs to the thick and rigid subcategory of Chow motives spanned by the motives of abelian varieties over $k$. This paper contains three sections of independent interest. First, we show that a motive which becomes of abelian type after a base field extension of algebraically closed fields is of abelian type. Given a field extension $K/k$ and a motive $M$ over $k$, we also show that $M$ is finite-dimensional if and only if $M_K$ is finite-dimensional. As a corollary, we obtain Chow--Kuenneth decompositions for varieties that become isomorphic to an abelian variety after some field extension. Second, let $\Omega$ be a universal domain containing $k$. We show that Murre's conjectures for motives of abelian type over $k$ reduce to Murre's conjecture (D) for products of curves over $\Omega$. In particular, we show that Murre's conjecture (D) for products of curves over $\Omega$ implies Beauville's vanishing conjecture on abelian varieties over $k$. Finally, we give criteria on Chow groups for a motive to be of abelian type. For instance, we show that $M$ is of abelian type if and only if the total Chow group of algebraically trivial cycles $CH_*(M_\Omega)_{alg}$ is spanned, via the action of correspondences, by the Chow groups of products of curves. We also show that a morphism of motives $f: N \to M$, with $N$ Kimura finite-dimensional, which induces a surjection $f_* : CH_*(N_\Omega)_{alg} \to CH_*(M_\Omega)_{alg}$ also induces a surjection $f_* : CH_*(N_\Omega)_{hom} \to CH_*(M_\Omega)_{hom}$ on homologically trivial cycles.