Higher Du Bois singularities of hypersurfaces

TL;DR

This paper introduces higher p-Du Bois singularities for complex hypersurfaces, showing their equivalence with higher p-log canonical singularities and providing explicit isomorphisms and improved bounds related to Bernstein-Sato polynomials.

Contribution

It generalizes the concept of Du Bois singularities to higher p, establishes their equivalence with higher p-log canonical singularities for hypersurfaces, and constructs explicit isomorphisms using Koszul complexes.

Findings

Higher p-Du Bois singularities coincide with higher p-log canonical singularities for hypersurfaces.

Explicit isomorphisms between sheaves of differential forms and Du Bois complexes are constructed.

Improved bounds on Bernstein-Sato polynomial roots are obtained for certain hypersurface singularities.

Abstract

For a complex algebraic variety $X$, we introduce higher $p$-Du Bois singularity by imposing canonical isomorphisms between the sheaves of K\"ahler differential forms $\Omega_X^q$ and the shifted graded pieces of the Du Bois complex $\underline{\Omega}_X^q$ for $q\le p$. If $X$ is a reduced hypersurface, we show that higher $p$-Du~Bois singularity coincides with higher $p$-log canonical singularity, generalizing a well-known theorem for $p=0$. The assertion that $p$-log canonicity implies $p$-Du Bois has been proved by Mustata, Olano, Popa, and Witaszek quite recently as a corollary of two theorems asserting that the sheaves of reflexive differential forms $\Omega_X^{[q]}$ ($q\le p$) coincide with $\Omega_X^q$ and $\underline{\Omega}_X^q$ respectively, and these are shown by calculating the depth of the latter two sheaves. We construct explicit isomorphisms between $\Omega_X^q$ and $\underline{\Omega}_X^q$ applying the acyclicity of a Koszul complex in a certain range. We also improve some non-vanishing assertion shown by them using mixed Hodge modules and the Tjurina subspectrum in the isolated singularity case. This is useful for instance to estimate the lower bound of the maximal root of the reduced Bernstein-Sato polynomial in the case where a quotient singularity is a hypersurface and its singular locus has codimension at most 4.