The stable hyperelliptic locus in genus 3: An application of Porteous Formula

We compute the class of the closure of the locus of hyperelliptic curves in the moduli space of stable genus-3 curves in terms of the tautological class λ   and the boundary classes δ0δ0 and δ1δ1. The expression of this class is known, but here we compute it directly, by means of Porteous Formula, without resorting to blowups or test curves.