Paper 1, Section II, I

Riemann Surfaces
Part II, 2013

(i) Let f(z)=n=0anznf(z)=\sum_{n=0}^{\infty} a_{n} z^{n} be a power series with radius of convergence rr in (0,)(0, \infty). Show that there is at least one point aa on the circle C={zC:z=r}C=\{z \in \mathbb{C}:|z|=r\} which is a singular point of ff, that is, there is no direct analytic continuation of ff in any neighbourhood of aa.

(ii) Let XX and YY be connected Riemann surfaces. Define the space G\mathcal{G} of germs of function elements of XX into YY. Define the natural topology on G\mathcal{G} and the natural mapπ:GX\operatorname{map} \pi: \mathcal{G} \rightarrow X. [You may assume without proof that the topology on G\mathcal{G} is Hausdorff.] Show that π\pi is continuous. Define the natural complex structure on G\mathcal{G} which makes it into a Riemann surface. Finally, show that there is a bijection between the connected components of G\mathcal{G} and the complete holomorphic functions of XX into YY.