Weierstrass weight of Gorenstein singularities with one or two branches

Abstract

Let $X$ denote an integral, projective Gorenstein curve over an algebraically closed field $k$. In the case when $k$ is of characteristic zero, C. Widland and the second author have defined Weierstrass points of a line bundle on $X$. In the first section, this definition is extended to linear systems in arbitrary characteristic. This definition may be viewed as a generalization of the definitions of Laksov and Stöhr-Voloch to the Gorenstein case. In the second section, an example is given to illustrate the definition. This example is a plane curve of arithmetic genus 3 in characteristic 2 such that the gap sequence at every smooth point (with respect to the dualizing bundle) is 1,2,5 and there are no smooth Weierstrass points. In the third section, the Weierstrass weight of a unibranch singularity (on a not necessarily rational curve) is computed in terms of its semigroup of values. In the final section, the Weierstrass weight of a singularity with precisely two branches (again assuming that the characteristic is zero) is computed. AMS Classification: Primary 14H55, Secondary 14H20.