On the deep structure of the blowing-up of curve singularities

Abstract

Let C be a germ of curve singularity embedded in (kⁿ, 0). It is well known that the blowing-up of C centred on its closed ring, Bl(C), is a finite union of curve singularities. If C is reduced we can iterate this process and, after a finite number of steps, we find only non-singular curves. This is the desingularization process. The main idea of this paper is to linearize the blowing-up of curve singularities Bl(C) → C. We perform this by studying the structure of [Oscr ]_Bl(C)/[Oscr ]_C as W-module, where W is a discrete valuation ring contained in [Oscr ]_C. Since [Oscr ]_Bl(C)/[Oscr ]_C is a torsion W-module, its structure is determined by the invariant factors of [Oscr ]_C in [Oscr ]_Bl(C). The set of invariant factors is called in this paper as the set of micro-invariants of C (see Definition 1·2).

In the first section we relate the micro-invariants of C to the Hilbert function of C (Proposition 1·3), and we show how to compute them from the Hilbert function of some quotient of [Oscr ]_C (see Proposition 1·4).

The main result of this paper is Theorem 3·3 where we give upper bounds of the micro-invariants in terms of the regularity, multiplicity and embedding dimension. As a corollary we improve and we recover some results of [6]. These bounds can be established as a consequence of the study of the Hilbert function of a filtration of ideals g = {g^[r,i+1]}_{i [ges ] 0} of the tangent cone of [Oscr ]_C (see Section 2). The main property of g is that the ideals g^[r,i+1] have initial degree bigger than the Castelnuovo–Mumford regularity of the tangent cone of [Oscr ]_C.

Section 4 is devoted to computation the micro-invariants of branches; we show how to compute them from the semigroup of values of C and Bl(C) (Proposition 4·3). The case of monomial curve singularities is especially studied; we end Section 4 with some explicit computations.

In the last section we study some geometric properties of C that can be deduced from special values of the micro-invariants, and we specially study the relationship of the micro-invariants with the Hilbert function of [Oscr ]_Bl(C). We end the paper studying the natural equisingularity criteria that can be defined from the micro-invariants and its relationship with some of the known equisingularity criteria.