Factoriality of normal projective varieties

TL;DR

This paper improves the understanding of the factoriality defect of normal projective varieties by refining topological formulas and establishing new conditions under which Q-factoriality implies factoriality, with implications for singularity theory.

Contribution

It provides a refined topological formula for the Q-factoriality defect under weaker assumptions and proves that Q-factoriality implies factoriality for certain local complete intersection varieties.

Findings

Improved topological formula for $f Q$-factoriality defect under 2-semi-rationality.

Q-factoriality implies factoriality for local complete intersections with codimension ≥ 3 singular locus.

Generalization of Grothendieck's theorem relating factoriality and singularities.

Abstract

For a normal projective variety $X$, the $\bf Q$-factoriality defect $\sigma(X)$ is defined to be the rank of the quotient of the group of Weil divisors by the subgroup of Cartier ones. We prove a slight improvement of a topological formula of S.G. Park and M. Popa asserting that $\sigma(X)=h^{2n-2}(X)-h^2(X)$ by assuming only 2-semi-rationality, that is, $R^k\pi_*{\mathcal O}_{\widetilde{X}}=0$ for $k=1,2$, instead of rational singularities for $X$, where $\pi:\widetilde{X}\to X$ is a desingularization with $h^k(X):=\dim H^k(X,{\bf Q})$ and $n:=\dim X>2$. Our proof generalizes the one by Y. Namikawa and J.H.M. Steenbrink for the case $n=3$ with isolated hypersurface singularities. We also give a proof of (a slight generalization of) the assertion that $\bf Q$-factoriality implies factoriality if $X$ is a local complete intersection whose singular locus has at least codimension three. These imply a slight improvement of Grothendieck's theorem in the projective case asserting that $X$ is factorial if it is a local complete intersection whose singular locus has at least codimension three and at general points of its components of codimension three, $X$ has rational singularities and is a $\bf Q$-homology manifold.