Separating semigroup of genus 4 curves

Abstract

A rational function on a real algebraic curve $C$ is called separating if it takes real values only at real points. Such a function defines a covering $\mathbb{R}C\to {\mathbb{RP}}^1$. Let $c_1,\dots ,c_r$ be connected components of $\mathbb{R}C$. M. Kummer and K. Shaw defined the separating semigroup of $C$ as the set of all sequences $(d_1(f),\dots ,d_r(f))$ where $f$ is a separating function and $d_i(f)$ is the degree of the restriction of $f$ to $c_i$. In the present paper we describe the separating semigroups of all genus 4 curves. For the proofs we consider the canonical embedding of $C$ into a quadric $X$ in ${\mathbb{P}}^3$ and apply Abel's theorem to 1-forms obtained as Poincaré residues at $C$ of certain meromorphic 2-forms on $X$.

10 pages. Version 2: numerous misprints are corrected and Section 3 is added