4.II.23F

Riemann Surfaces
Part II, 2007

Let RR be a Riemann surface, R~\widetilde{R}a topological surface, and p:R~Rp: \widetilde{R} \rightarrow R a continuous map. Suppose that every point xR~x \in \widetilde{R}admits a neighbourhood U~\widetilde{U}such that pp maps U~\widetilde{U}homeomorphically onto its image. Prove that R~\widetilde{R}has a complex structure such that pp is a holomorphic map.

A holomorphic map π:YX\pi: Y \rightarrow X between Riemann surfaces is called a covering map if every xXx \in X has a neighbourhood VV with π1(V)\pi^{-1}(V) a disjoint union of open sets WkW_{k} in YY, so that π:WkV\pi: W_{k} \rightarrow V is biholomorphic for each WkW_{k}. Suppose that a Riemann surface YY admits a holomorphic covering map from the unit disc{zC:z<1}\operatorname{disc}\{z \in \mathbb{C}:|z|<1\}. Prove that any holomorphic map CY\mathbb{C} \rightarrow Y is constant.

[You may assume any form of the monodromy theorem and basic results about the lifts of paths, provided that these are accurately stated.]