Abrahamseʹs interpolation theorem and Fuchsian groups

We generalize Abrahamseʹs interpolation theorem from the setting of a multiply connected domain to that of a more general Riemann surface. Our main result provides the scalar-valued interpolation theorem for the fixed-point subalgebra of H ∞ associated to the action of a Fuchsian group. We rely on two results from a paper of Forelli. This allows us to prove the interpolation result using duality techniques that parallel Sarasonʹs approach to the interpolation problem for H ∞ [Donald Sarason, Generalized interpolation in H ∞ , Trans. Amer. Math. Soc. 127 (1967) 179–203, MR0208383. [26]]. In this process we prove a more general distance formula, very much like Nehariʹs theorem, and obtain relations between the kernel function for the character automorphic Hardy spaces and the Szegö kernel for the disk. Finally, we examine our interpolation results in the context of the two simplest examples of Fuchsian groups acting on the disk.