Smoothings and Rational Double Point Adjacencies for Cusp Singularities

TL;DR

This paper investigates the deformation and adjacency properties of cusp surface singularities, providing a generalized classification and explicit constructions for their smoothings and degenerations, extending previous results to arbitrary cusp cases.

Contribution

It generalizes the classification of rational double point configurations for cusp singularities and constructs explicit semistable resolutions for all cases.

Findings

Extended classification to arbitrary cusp singularities.

Constructed explicit semistable resolutions.

Analyzed monodromy and deformation properties.

Abstract

A cusp singularity is a surface singularity whose minimal resolution is a cycle of smooth rational curves meeting transversely. Cusp singularities come in naturally dual pairs. Looijenga proved in 1981 that if a cusp singularity is smoothable, the minimal resolution of the dual cusp is the anticanonical divisor of some smooth rational surface. In 1983, the second author and Miranda gave a criterion for smoothability of a cusp singularity, in terms of the existence of a K-trivial semistable model for the central fiber of such a smoothing. We study these "Type III degenerations" of rational surfaces with an anticanonical divisor--their deformations, birational geometry, and monodromy. Looijenga's original paper also gave a description of the rational double point configurations to which a cusp singularity deforms, but only in the case where the resolution of the dual cusp has cycle length 5 or less. We generalize this classification to an arbitrary cusp singularity, giving an explicit construction of a semistable simultaneous resolution of such an adjacency. The main tools of the proof are (1) formulas for the monodromy of a Type III degeneration, (2) a construction via surgeries on integral-affine surfaces of a degeneration with prescribed monodromy, (3) surjectivity of the period map for Type III central fibers, and (4) a theorem of Shepherd-Barron producing the simultaneous contraction to the adjacency of the cusp singularity.