On a subvariety of the moduli space

Abstract

We give an explicit description of a non-normal irreducible subvariety of the moduli space of Riemann surfaces of genus $3$ characterized by a non-cyclic group action. Defining equations of a family of curves representing non-normal points of this subvariety are computed. We also find defining equations of the family of hyperelliptic curves of genus $3$ whose full automorphism group is $C_2\times C_4$. This completes the list of full automorphism groups of hyperelliptic curves.