Homology group automorphisms of Riemann surfaces

Abstract

If $Γ$ is a finitely generated Fuchsian group such that its derived subgroup $Γ'$ is co-compact and torsion free, then $S={\mathbb H}^{2}/Γ'$ is a closed Riemann surface of genus $g \geq 2$ admitting the abelian group $A=Γ/Γ'$ as a group of conformal automorphisms. We say that $A$ is a homology group of $S$. A natural question is if $S$ admits unique homology groups or not, in other words, is there are different Fuchsian groups $Γ_{1}$ and $Γ_{2}$ with $Γ_{1}'=Γ'_{2}$? It is known that if $Γ_{1}$ and $Γ_{2}$ are both of the same signature $(0;k,\ldots,k)$, for some $k \geq 2$, then the equality $Γ_{1}'=Γ_{2}'$ ensures that $Γ_{1}=Γ_{2}$. Generalizing this, we observe that if $Γ_{j}$ has signature $(0;k_{j},\ldots,k_{j})$ and $Γ_{1}'=Γ'_{2}$, then $Γ_{1}=Γ_{2}$. We also provide examples of surfaces $S$ with different homology groups. A description of the normalizer in ${\rm Aut}(S)$ of each homology group $A$ is also obtained.