Threefolds and deformations of surface singularities

Abstract

The central theme of this article is the study of deformations of surface singularities using recent advances in three dimensional geometry. The basic idea is the following. Let X 0 be a surface singularity and consider a one parameter deformation {Xo: t e A } . Then the total space X = U X t is a three dimensional object. One can attempt to use the geometry of X to get information about the surface X~. In general X is very singular and so one can try to study it via a suitable resolution of singularities f : X ' -~ X. The existence of a resolution was established by Zariski; the problem is that there are too many of them, none particularly simple. Mori and Reid discovered that the best one can hope for is a partial resolution f : X ' ~ X where X' possesses certain mild singularities but otherwise is a good analog of the minimal resolution of surface singularities. The search for such a resolution is known as Mori 's program (see e.g. [-Ko3, KMM]). After substantial contributions by several mathematicians (Benveniste, Kawamata, Kollfir, Mori, Reid, Shokurov, Vichweg) this was recently completed by Mori [Mo 3]. A special case, which is nonetheless sufficient for the applications presented here, was settled by several persons. A proof was first announced by Tsunoda [TsM], later followed by Shokurov [Sh], Mori [Mo2] and Kawamata [Kaw2]. A precise formulation of the result we need will be provided at the end of the introduction. In certain situations X0 will impose very strong restrictions on X ' and one can use this to obtain information about X and X~ for t 40 . The first application is in chapter two. Teissier [Tel posed the following problem. Let {X~ : s~S} be a flat family of surfaces parameterized by the connected space S. Let X s be the minimal resolution of X~. In general {Xs: s e S } is not a flat family of surfaces, and it is of interest to find necessary and sufficient conditions for this to hold.