The $a$-numbers of non-hyperelliptic curves of genus 3 with cyclic automorphism group of order 6

Abstract

In this paper, we study non-hyperelliptic curves of genus $3$ with cyclic automorphism group of order $6$. Over an algebraically closed field $K$ of characteristic $\ne 2,3$, such curves are written as plane quartics $C_r:x^{3}z+y^4+r{y}^2{z}^2+z^4=0$ with one parameter $r$. As the first main theorem, we show that $r\ne 0,\pm 2$ and give a necessary and sufficient condition with respect to $r$ and $r^{\prime }$ such that $C_r\cong C_{r^{\prime }}$. By describing the Hasse-Witt matrix of $C_r$ in terms of a certain Gauss' hypergeometric series, we obtain the second main theorem, where we determine the possible $a$-number of $C_r$, and give the exact number of isomorphism classes over $K$ of such curves attaining the possible maximal $a$-number.

23 pages, to appear in Acta Arithmetica