English

A Lefschetz theorem for intersections with projective varieties

Algebraic Geometry 2020-05-22 v1 Algebraic Topology Number Theory

Abstract

One version of the classical Lefschetz hyperplane theorem states that for UPnU \subset \mathbb P^n a smooth quasi-projective variety of dimension at least 22, and HUH \cap U a general hyperplane section, the resulting map on \'etale fundamental groups π1(HU)π1(U)\pi_1(H \cap U) \rightarrow \pi_1(U) is surjective. We prove a generalization, replacing the hyperplane by a general PGLn+1\operatorname{PGL}_{n+1}-translate of an arbitrary projective variety: If UPnU \subset \mathbb P^n is a normal quasi-projective variety, XX is a geometrically irreducible projective variety of dimension at least n+1dimUn + 1 - \dim U, and YY is a general PGLn+1\operatorname{PGL}_{n+1}-translate of XX, then the map π1(YU)π1(U)\pi_1(Y \cap U) \rightarrow \pi_1(U) is surjective.

Keywords

Cite

@article{arxiv.2005.10708,
  title  = {A Lefschetz theorem for intersections with projective varieties},
  author = {Aaron Landesman},
  journal= {arXiv preprint arXiv:2005.10708},
  year   = {2020}
}
R2 v1 2026-06-23T15:43:09.168Z