On the locus of curves with an odd subcanonical marked point

Abstract

We present an explicit construction of a compactification of the locus of smooth curves whose symmetric Weierstrass semigroup at a marked point is odd. The construction is an extension of Stoehr's techniques using Pinkham'sequivariant deformation of monomial curves by exploring syzygies. As an application we prove the rationality of the locus for genus at most six.