A class of pseudoreal Riemann surfaces with diagonal automorphism group

Abstract

A Riemann surface $\mathcal{S}$ having field of moduli $\mathbb{R}$, but not a field of definition, is called \emph{pseudoreal}. This means that $\mathcal{S}$ has anticonformal automorphisms, but non of them is an involution. We call a Riemann surface $\mathcal{S}$ \emph{plane} if it can be described by a smooth plane model of some degree $d\geq4$ in $\mathbb{P}^2_{\mathbb{C}}$. We characterize pseudoreal-plane Riemann surfaces $\mathcal{S}$, whose conformal automorphism group $\operatorname{Aut}_+(\mathcal{S})$ is $\operatorname{PGL}_3(\mathbb{C})$-conjugate to a finite non-trivial group $G$ that leaves invariant infinitely many points of $\mathbb{P}^2_{\mathbb{C}}$. In particular, we show that such pseudoreal-plane Riemann surfaces exist only if $\operatorname{Aut}_+(\mathcal{S})$ is cyclic of even order $n$ dividing the degree $d$. Explicit examples are given, for any degree $d=2pm$ with $m>1$ odd, $p$ is prime and $n=d/p$.