Infinitesimal extensions of ℙ1 and their Hilbert schemes

Abstract.

 In order to calculate the multiplicity of an isolated rational curve C on a local complete intersection variety X, i.e. the length of the local ring of the Hilbert Scheme of X at [C], it is important to study infinitesimal neighborhoods of the curve in X. This is equivalent to infinitesimal extensions of ℙ¹ by locally free sheaves. In this paper we study infinitesimal extensions of ℙ¹, determine their structure and obtain upper and lower bounds for the length of the local rings of their Hilbert schemes at [ℙ¹].