Detecting effectivity of motives, their weights, connectivity, and dimension via Chow-weight (co)homology: a "mixed motivic decomposition of the diagonal"

Abstract

We describe certain criteria for a motif $M$ to be $r$-effective, i.e., to belong to the $r$th Tate twist $Obj DM^{eff}_{gm,R}(r)=Obj DM^{eff}_{gm,R} \otimes L^{\otimes r}$ of effective Voevodsky motives (for $r\ge 1$; $R$ is the coefficient ring). In particular, $M$ is 1-effective if and only if a complex whose terms are certain Chow groups of zero-cycles is acyclic. The dual to this statement checks whether an effective motif $M$ belongs to the subcategory of $DM^{eff}_{gm,R}$ generated by motives of varieties of dimension $\le r$. These criteria are formulated in terms of the Chow-weight (co)homology of $M$. These (co)homology theories are introduced in the current paper and have several (other) remarkable properties: they yield a bound on the "weights" of $M$ (in the sense of the Chow weight structure defined by the first author) and detect the effectivity of "the lower weight pieces" of $M$. We also calculate the "connectivity" of $M$ (in the sense of Voevodsky's homotopy t-structure) and prove that the exponents of the higher motivic homology groups (of an "integral" motif) are bounded whenever these groups are torsion. These motivic properties of $M$ have important consequences for its cohomology. As a corollary, we prove that if Chow groups of an arbitrary variety $X$ vanish up to dimension $r-1$ then the highest Deligne weight factors of the (singular or \'etale) cohomology of $X$ with compact support are $r$-effective in the naturally defined sense. Our results yield a vast generalization of the so-called "decomposition of the diagonal" statements.