Semi‐Arithmetic Fuchsian Groups and Modular Embeddings

Arithmetic Fuchsian groups are the most interesting and most important Fuchsian groups owing to their significance for number theory and owing to their geometric properties. However, for a fixed signature there exist only finitely many non‐conjugate arithmetic Fuchsian groups; it is therefore desirable to extend this class of Fuchsian groups. This is the motivation of our definition of semi‐arithmetic Fuchsian groups. Such a group may be defined as follows (for the precise formulation see Section 2). Let Γ be a cofinite Fuchsian group and let Γ 2 be the subgroup generated by the squares of the elements of Γ. Then Γ is semi‐arithmetic if Γ is contained in an arithmetic group Δ acting on a product H r of upper halfplanes. Equivalently, Γ is semi‐arithmetic if all traces of elements of Γ 2 are algebraic integers of a totally real field. Well‐known examples of semi‐arithmetic Fuchsian groups are the triangle groups (and their subgroups of finite index) which are almost all non‐arithmetic with the exception of 85 triangle groups listed by Takeuchi [ 16 ]. While it is still an open question as to what extent the non‐arithmetic Fuchsian triangle groups share the geometric properties of arithmetic groups, it is a fact that their automorphic forms share certain arithmetic properties with modular forms for arithmetic groups. This has been clarified by Cohen and Wolfart [ 5 ] who proved that every Fuchsian triangle group Γ admits a modular embedding, meaning that there exists an arithmetic group Δ acting on H r , a natural group inclusion f :Γ→Δ and a compatible holomorphic embedding F :H→H r that is with F ( yZ )= f ( y ) F ( z ) for all γ∈Γ and all z ∈H.