English

Weak approximation for Fano complete intersections in positive characteristic

Algebraic Geometry 2018-12-31 v2

Abstract

For a smooth curve BB over an algebraically closed field kk, for every BB-flat complete intersection XBX_B in B×Spec kPknB\times_{\text{Spec}\ k} \mathbb{P}^n_k of type (d1,,dc)(d_1,\dots,d_c), if the Fano index is 2\geq 2 and if char(k)>max(d1,,dc)\text{char}(k)>\max(d_1,\dots,d_c), we prove weak approximation of O^B,b\widehat{\mathcal{O}}_{B,b}-points of XBX_B by k(B)k(B)-points at all places of (strong) potentially good reduction, including all places of good reduction. The key step is the proof that such complete intersections are \emph{separably uniruled by lines}, and even \emph{separably rationally connected}, whenever smooth. We prove that the inequality is close to sharp. We prove a similar theorem for Fano manifolds of Picard number 11 and Fano index 11.

Keywords

Cite

@article{arxiv.1811.02466,
  title  = {Weak approximation for Fano complete intersections in positive characteristic},
  author = {Jason Michael Starr and Zhiyu Tian and Runhong Zong},
  journal= {arXiv preprint arXiv:1811.02466},
  year   = {2018}
}

Comments

22 pages. Following discussions with Shizhang Li and Bhargav Bhatt, added hypothesis about torsion in crystalline cohomology for specializations of complex Fano manifolds of Picard number $1$ and Fano index $1$. Also proved a version eliminating this hypothesis and the hypothesis that the mixed characteristic DVR is unramified

R2 v1 2026-06-23T05:06:35.469Z