Holomorphic mappings of finitely connected planar domains

Abstract

Results on holomorphic mappings of finitely connected planar domains are often proved using Koebe's famous circle mapping theorem. A prominent example is Julia's bound on the size of the automorphism group of finitely connected planar domains of connectivity $k\geq3$ and the sharp bound obtained later by Heins. The purpose of this article is to explore whether it is possible to give a proof of these results, and several other related results, without using Koebe's theorem. The main result is a rigidity theorem for proper holomorphic maps between finitely connected domains of connectivity higher than $2$ that immediately yields a bound on the number of proper holomorphic mappings between two such domains in terms of their connectivities. Using the same ideas, we also provide a new elementary proof of the planar Riemann--Hurwitz formula that does not use the Euler characteristic. Most of our proofs rely only on the Riemann mapping theorem, the Schwarz reflection principle, and the argument principle.