On finiteness properties of separating semigroup of real curve
Authors
Matthew Magin
Abstract
A real morphism $f$ from a real algebraic curve $X$ to $\mathbb{P}^1$ is called \emph{separating} if $f^{-1}(\mathbb{R} \mathbb{P}^1) = \mathbb{R} X$. A separating morphism defines a covering $\mathbb{R} X \to \mathbb{R} \mathbb{P}^1$. Let $X_1, \ldots, X_r$ denote the components of $\mathbb{R} X$. M. Kummer and K. Shaw~\cite{kummer_separating_2020} defined the separating semigroup of a curve $X$ as the set of all vectors $d(f) = (d_1(f), \ldots, d_r(f)) \in \mathbb{N}^{r}$ where $f$ is a separating morphism $X \to \mathbb{P}^1$ and $d_i(f)$ is the degree of the restriction of $f$ to $X_i$.
Let us call an additive subsemigroup of $\mathbb{N}^{r}$ \emph{finitely covered} if it can be written as $S = S_0 \cup \bigcup_{i=1}^{m} (s_i + \mathbb{N}_{0}^{r})$, where $S_0$ is a finite set. In the present paper, we prove that the separating semigroup of a real curve $X$ is finitely covered, but not finitely generated when $\mathbb{R} X$ has at least two connected components.
