On the fiber product of Riemann surfaces

TL;DR

This paper studies the fiber product of Riemann surfaces, providing a Fuchsian description, analyzing irreducibility, and exploring the structure of the Jacobian when the fiber product is connected.

Contribution

It offers a new Fuchsian approach to fiber products, characterizes irreducibility conditions, and examines the field of moduli and Jacobian decompositions.

Findings

Irreducible components are isomorphic if one map is a regular branched cover.

Number of irreducible components is bounded by the gcd of degrees.

Connected fiber products have a decomposable Jacobian.

Abstract

Let $S_{0}, S_{1}$ and $S_{2}$ be connected Riemann surfaces and let $\beta_{1}:S_{1} \to S_{0}$ and $\beta_{2}:S_{2} \to S_{0}$ be surjective holomorphic maps. The associated fiber product $S_{1} \times_{(\beta_{1},\beta_{2})} S_{2}$ has the structure of a singular Riemann surface, endowed with a canonical map $\beta$ to $S_{0}$ satisfying that $\beta_{j} \circ \pi_{j}=\beta$, where $\pi_{j}$ is coordinate projection onto $S_{j}$. In this paper we provide a Fuchsian description of the fiber product and obtain that if one the maps $\beta_{j}$ is a regular branched cover, then all its irreducible components are isomorphic. In the case that both $\beta_{j}$ are of finite degree, we observe that the number of irreducible components is bounded above by the greatest common divisor of the two degrees; we study the irreducibility of the fiber product. In the case that $S_{0}=\widehat{\mathbb C}$, and $S_{1}$ and $S_{2}$ are compact, we define the strong field of moduli of the pair $(S_{1} \times_{(\beta_{1},\beta_{2})} S_{2},\beta)$ and observe that this field coincides with the minimal field containing the fields of moduli of both pairs $(S_{1},\beta_{1})$ and $(S_{2},\beta_{2})$. Finally, in the case that the fiber product is a connected Riemann surface, we provide an isogenous decomposition of its Jacobian variety.