Factoriality of normal projective varieties
Seung-Jo Jung, Morihiko Saito
TL;DR
The paper refines the topological understanding of the Q-factoriality defect σ(X) for normal projective varieties by replacing rational singularity assumptions with 2-semi-rationality and employing mixed Hodge module techniques. It proves the improved formula σ(X) = h^{2n−2}(X) − h^2(X) under 2-semi-rationality and shows stability of the relevant Hodge-theoretic data, extending Namikawa–Steenbrink’s n = 3 isolated singularity result to higher dimensions. It also clarifies when Q-factoriality implies factoriality for local complete intersections with sufficiently high codimension of the singular locus, yielding a projective Grothendieck-style factoriality criterion and a Cohen–Macaulay–variant that cannot replace factoriality by Q-factoriality in general. The results connect divisor-class data to intersection cohomology via desingularizations, weight filtrations, and vanishing results, with implications for longstanding questions about the structure of singularities in projective varieties.
Abstract
For a normal projective variety $X$, the $\bf Q$-factoriality defect $σ(X)$ 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 $σ(X)=h^{2n-2}(X)-h^2(X)$ by assuming only 2-semi-rationality, that is, $R^kπ_*{\mathcal O}_{\widetilde{X}}=0$ for $k=1,2$, instead of rational singularities for $X$, where $π:\widetilde{X}\to X$ 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 the assertion that $\bf Q$-factoriality implies factoriality if $X$ is a local complete intersection whose singular locus has at least codimension three. (This seems to be known to specialists in the case $X$ has only isolated hypersurface singularities with $n=3$ using Milnor's Bouquet theorem.) These imply another proof of Grothendieck's theorem in the projective case asserting that $X$ is factorial if $X$ is a local complete intersection whose singular locus has at least codimension four. We can also prove a variant with factorial and local complete intersection replaced respectively by $\bf Q$-factorial and Cohen-Macaulay, where $\bf Q$-factorial cannot be replaced by factorial.