Specialization of N\'eron-Severi groups in characteristic $p$

Abstract

Andr\'{e} and Maulik--Poonen proved that for any smooth proper family $X\to B$ of varieties over an algebraically closed field of characteristic $0$, there is a closed fiber whose N\'{e}ron-Severi group has the same rank as that of the N\'{e}ron-Severi group of the geometric generic fiber. We prove the analogous statement over algebraically closed fields of characteristic $p>0$ which are not isomorphic to $\overline{\mathbb{F}}_p$. Furthermore, we prove that for any algebraically closed field $k$ of characteristic $p>0$ and smooth proper family $X\to B$ of $k$-varieties, there exists a dense open subvariety $U\subseteq B$ and integer $N$ such that for each map $x:\mathrm{Spec}\, k[[t]]\to U$, the $p$-torsion in the cokernel of the specialization map from the N\'{e}ron-Severi group of the pullback of $X$ to the geometric generic fiber of $x$ to the N\'{e}ron-Severi group of the pullback of $X$ to the special fiber of $x$ is killed by $p^N$. Finally, we prove that for a curve $C$ over $k$ and family $\mathscr{X}\to\mathscr{B}$ of smooth $C$-schemes, there exists a dense Zariski open $U\subseteq \mathscr{B}$ such that for a local uniformizer $t$ at any closed point of $C$, the rank of the N\'{e}ron-Severi group jumps only on a $t$-adic nowhere dense set $t$. The crystalline Lefschetz $(1,1)$ theorem of Morrow is a key ingredient in the proofs.