Paper 3, Section II, F

Riemann Surfaces
Part II, 2017

Let n2n \geqslant 2 be a positive even integer. Consider the subspace RR of C2\mathbb{C}^{2} given by the equation w2=zn1w^{2}=z^{n}-1, where (z,w)(z, w) are coordinates in C2\mathbb{C}^{2}, and let π:RC\pi: R \rightarrow \mathbb{C} be the restriction of the projection map to the first factor. Show that RR has the structure of a Riemann surface in such a way that π\pi becomes an analytic map. If τ\tau denotes projection onto the second factor, show that τ\tau is also analytic. [You may assume that RR is connected.]

Find the ramification points and the branch points of both π\pi and τ\tau. Compute the ramification indices at the ramification points.

Assume that, by adding finitely many points, it is possible to compactify RR to a Riemann surface Rˉ\bar{R} such that π\pi extends to an analytic map πˉ:RˉC\bar{\pi}: \bar{R} \rightarrow \mathbb{C}_{\infty}. Find the genus of Rˉ\bar{R} (as a function of nn ).