Bertini theorems for Hilbert-Samuel multiplicity over finite fields

Abstract

Let $X$ be a quasiprojective subscheme of $\mathbb{P}^n_{\mathbb{F}_q}$. We prove a Bertini theorem for Hilbert--Samuel multiplicity of $X$; that is, there exists a positive-density set of hypersurfaces $H_f$ such that for every point $ξ\in X\cap H_f$, one has $\operatorname{ord}_ξ(f)=1$ and $e_ξ(X\cap H_f)=e_ξ(X)$. Furthermore, we extend this result on hypersurfaces to complete intersections, hypersurfaces containing a prescribed subscheme, and semiample linear systems.