Néron--Severi groups of proper schemes over finite fields

Abstract

Let \(X\) be a proper reduced scheme over a finite field \(k\), let \(\ell\) be a prime different from \(\operatorname{char} k\), and write \(\ol X=X\times_{k}\ol k\) for its base change to an algebraic closure \(\ol k\) of \(k\). Call a class in \(\rH^{2}_{\et}(\ol X,\bbZ_{\ell}(1))\) \emph{Zariski-locally trivial} if it vanishes on a Zariski-open cover of \(\ol X\). We prove that the first Chern class map identifies \(\NS(\ol X)\otimes\bbZ_{\ell}\) with the group of Zariski-locally trivial classes whose image in \(\rH^{2}_{\et}(\ol X,\Ql(1))\) has weight zero. This is the finite-field analogue of a theorem of Barbieri-Viale--Rosenschon--Srinivas for proper seminormal complex varieties. In the finite-field setting neither seminormality nor irreducibility is needed.