Smoothable locally Cohen--Macaulay and non Cohen--Macaulay multiple structures on curves

Abstract

In this article we show that a wide range of multiple structures on curves arise whenever a family of embeddings degenerates to a morphism $φ$ of degree $n$. One could expect to see, when an embedding degenerates to such a morphism, the appearance of a locally Cohen-Macaulay multiple structure of certain kind (a so-called rope of multiplicity $n$). We show that this expectation is naive and that both locally Cohen-Macaulay and non Cohen-Macaulay multiple structures might occur in this situation. In seeing this we find out that many multiple structures can be smoothed. When we specialize to the case of double structures we are able to say much more. In particular, we find numerical conditions, in terms of the degree and the arithmetic genus, for the existence of many smoothable double structures. Also, we show that the existence of these double structures is determined, although not uniquely, by the elements of certain space of vector bundle homomorphisms, which are related to the first order infinitesimal deformations of $φ$. In many instances, we show that, in order to determine a double structure uniquely, looking merely at a first order deformation of $φ$ is not enough; one needs to choose also a formal deformation.