English

Failure of Lefschetz hyperplane theorem

Algebraic Geometry 2023-01-13 v1

Abstract

In this article, we give a counterexample to the Lefschetz hyperplane theorem for non-singular quasi-projective varieties. A classical result of Hamm-L\^{e} shows that Lefschetz hyperplane theorem can hold for hyperplanes in general position. We observe that the condition of ``hyperplane'' is strict in the sense that it is not possible to replace it by higher degree hypersurfaces. The counterexample is very simple: projective space minus finitely many points. Moreover, as an intermediate step we prove that the Grothendieck-Lefschetz theorem also fails in the quasi-projective case.

Cite

@article{arxiv.2301.04940,
  title  = {Failure of Lefschetz hyperplane theorem},
  author = {Ananyo Dan},
  journal= {arXiv preprint arXiv:2301.04940},
  year   = {2023}
}

Comments

5 pages

R2 v1 2026-06-28T08:10:08.204Z