Paper 1, Section II, 23G

Riemann Surfaces
Part II, 2011

Suppose that R1R_{1} and R2R_{2} are Riemann surfaces, and AA is a discrete subset of R1R_{1}. For any continuous map α:R1R2\alpha: R_{1} \rightarrow R_{2} which restricts to an analytic map of Riemann surfaces R1\AR2R_{1} \backslash A \rightarrow R_{2}, show that α\alpha is an analytic map.

Suppose that ff is a non-constant analytic function on a Riemann surface RR. Show that there is a discrete subset ARA \subset R such that, for PR\A,fP \in R \backslash A, f defines a local chart on some neighbourhood of PP.

Deduce that, if α:R1R2\alpha: R_{1} \rightarrow R_{2} is a homeomorphism of Riemann surfaces and ff is a non-constant analytic function on R2R_{2} for which the composite fαf \circ \alpha is analytic on R1R_{1}, then α\alpha is a conformal equivalence. Give an example of a pair of Riemann surfaces which are homeomorphic but not conformally equivalent.

[You may assume standard results for analytic functions on domains in the complex plane.]