Variation of Mixed Hodge Structures associated to an equisingular one-dimensional family of Calabi-Yau threefolds

Abstract

We study the Variations of mixed Hodge structures (VMHS) associated to a pencil ${\cal X}$ (parametrised by an open set $B \subset {\Bbb P}^1$) of equisingular hypersurfaces of degree $d$ in ${\Bbb P}^{4}$ with exactly $m$ ordinary double points as singularities as well as the variations of Hodge structures (VHS) associated to the desingularization of this family $ \widetilde{\cal X}$. The case where exactly $l \le m $ of those double points are in algebraic general position (short:agp) is studied in detail and determine the possible limiting mixed Hodge structures (LMHS) associated to each of the points in ${\Bbb P}^1\backslash B$. We find that the position of the singular points being in agp is not sufficient to describe the space of first one-adjoint conditions and naturally the notion of a set of singular points being in homologically good position (short: hg) is introduced. By requiring that the set of nodes in agp is also in hg, the $F^2$-term of the Hodge filtration of the desingularization is completely determined. The particular pencil $ {\cal X}$ of quintic hypersurfaces with $100$ singular double points with $86$ of them in agp which served as the starting point for this paper is treated with particular attention.