Deformations of hypersurfaces with non-constant Alexander polynomial

Abstract

Let X be an irreducible hypersurface in ${\mathbb{P}}^n$ of degree $d\ge 3$ with only isolated semi-weighted homogeneous singularities, such that $exp(\frac{2\pi i}{k})$ is a zero of the Alexander polynomial. Then we show that the equianalytic deformation space of $X$ is not $T$-smooth except for a finite list of triples $(n,d,k)$. This result captures the very classical examples by B. Segre of families of degree $6m$ plane curves with $6m^2$, $7m^2$, $8m^2$ and $9m^2$ cusps, where $m\ge 3$. Moreover, we argue that many of the hypersurfaces with non-trivial Alexander polynomial are limits of constructions of hypersurfaces with not $T$-smooth deformation spaces. In many instances this description can be used to construct Alexander-equivalent Zariski pairs.

Compared to v1: The main theorem has been slightly improved. Several minor changes. Section 2 rewritten Compared to v2: minor changes

References in corpus (2)

• The Spectrum of Hypersurface Singularities
• Mordell-Weil groups of elliptic threefolds and the Alexander module of plane curves