On the birational motive of hyper-K\"ahler varieties

TL;DR

This paper introduces a new filtration on Chow groups of hyper-K"ahler varieties, conjectures its relation to Voisin's filtration, and verifies this in key cases, advancing understanding of their birational motives.

Contribution

It proposes the co-radical filtration on Chow groups, establishes its equivalence with Voisin's filtration in specific cases, and links birational motives to surface decompositions and Beauville's eigenspaces.

Findings

The co-radical filtration matches Voisin's filtration for certain hyper-K"ahler varieties.

Birational motives of some hyper-K"ahler manifolds are determined by surface motives.

The co-radical filtration relates to Beauville's eigenspace decomposition on abelian varieties.

Abstract

We introduce a new ascending filtration, that we call the co-radical filtration in analogy with the basic theory of co-algebras, on the Chow groups of pointed smooth projective varieties. In the case of zero-cycles on projective hyper-K\"ahler manifolds, we conjecture it agrees with a filtration introduced by Voisin. This is established for moduli spaces of stable objects on K3 surfaces, for generalized Kummer varieties and for the Fano variety of lines on a smooth cubic fourfold. Our overall strategy is to view the birational motive of a smooth projective variety as a co-algebra object with respect to the diagonal embedding and to show in the aforementioned cases the existence of a so-called strict grading whose associated filtration agrees with the filtration of Voisin. As results of independent interest, we upgrade to rational equivalence Voisin's notion of "surface decomposition" and show that the birational motive of some projective hyper-K\"ahler manifolds is determined, as a co-algebra object, by the birational motive of a surface. We also relate our co-radical filtration on the Chow groups of abelian varieties to Beauville's eigenspace decomposition.