Canonical singularities of dimension three in characteristic 2 which do not follow Reid’s rules

摘要

We continue studying compound Du Val singularities defined over an algebraically closed field k, and present concrete examples in characteristic 2 which have onedimensional singular loci but do not admit a description as a trivial product (a rational double point) × (a curve) up to analytic isomorphism at any point. Unlike in other characteristics, we find a large number of such examples whose general hyperplane sections have rational double points of typeD. These compound Du Val singularities shall be viewed as a special class of canonical singularities. In the previous work with Ito and Saito,we classified such singularities in p ≥ 3, and I intend to complete our classification in arbitrary characteristic, reinforcing Reid’s result in characteristic 0.