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 x∈R~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:Y→X\pi: Y \rightarrow X between Riemann surfaces is called a covering map if every x∈Xx \in X has a neighbourhood VV with π−1(V)\pi^{-1}(V) a disjoint union of open sets WkW_{k} in YY, so that π:Wk→V\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⁡{z∈C:∣z∣<1}\operatorname{disc}\{z \in \mathbb{C}:|z|<1\}. Prove that any holomorphic map C→Y\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.]

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