Defect of projective hypersurfaces with isolated singularities

Abstract

Let $X$ be a hypersurface with isolated singularities defined by $f$ in ${\mathbf{P}}^{\mathbf{n}+1}$ with $n>1$. The difference $\mathrm{def}(X):=h^{n+1}(X)-h^{n-1}(X)$ is called the defect of $X$ (for self-duality of the cohomology of $X$). It is known that its vanishing is closely related to $\mathbf{Q}$-factoriality of $X$ without assuming rational singularities when $n=3$. This number coincides with the dimension of the cokernel of the inclusion $H^{n-1}(X)\to {\mathrm{IH}}^{n-1}(X)$, the rank of the morphism from the vanishing cohomologies of $X$ to $H^{n+1}(X)$ for a one-parameter smoothing of $X$ with total space smooth, and also with the dimension of the unipotent monodromy part of the Milnor fiber cohomology of $f$ with degree $n$. In the case $X$ has only weighted homogeneous isolated singularities, the defect $\mathrm{def}(X)$ is then given by the $E_2$-term of the spectral sequence of the double complex with differentials $\mathrm{d}f\wedge$ and $\mathrm{d}$ by the $E_2$-degeneration of the pole order spectral sequence. It can be calculated explicitly using a computer even for analogues of the Hirzebruch quintic threefold with more than one hundred ordinary double points found by B.\ van Geemen and J.\ Werner in a compatible way with their computation. We give also an example with $\mathrm{def}(X)>0$ and $|\mathrm{Sing}X|=1$ in the non-projective cone case where $n=3$.

Section 4 became an independent paper