A coproduct obstruction for derived unramified cohomology

Abstract

Let \(k\) be a perfect field of exponential characteristic \(p\), and let \(R\) be a commutative \(\Z[1/p]\)-algebra. We prove that the first derived unramified functor \[ F\longmapsto R^1_{\mathrm{nr},R}F \] from homotopy invariant Nisnevich sheaves with transfers of \(R\)-modules to birational sheaves commutes with arbitrary small direct sums. This gives a positive answer, after inverting the exponential characteristic, to a question of Kahn and Sujatha; on smooth projective varieties no inversion is needed. We also describe an obstruction to this for the functor $R^2_{\mathrm{nr},R}$ in categorical terms, which includes the familiar Griffiths group obstruction. As applications of the motivic nature of the functors \(R^q_{\mathrm{nr}}\), we prove torsion-order bounds and a correspondence-detection statement for surfaces.