Minimal rational graphs admitting a QHD smoothing

Abstract

Using the picture deformation technique of De Jong-Van Straten we show that no singularity whose resolution graph has 3 or 4 large nodes, i.e., nodes satisfying d(v)+e(v)\leq -2, has a QHD smoothing. This is achieved by providing a general reduction algorithm for graphs with QHD smoothings, and enumeration. New examples and families are presented, which admit a combinatorial QHD smoothing, i.e. the incidence relations for a sandwich presentation can be satisfied. We also give a new proof of the Bhupal-Stipsicz theorem on the classification of weighted homogeneous singularities admitting QHD smoothings with this method by using cusp singularities.

23 pages, 15 figures, final version, accepted to Pacific Journal of Mathematics