3.II.22 F3 . \mathrm{II} . 22 \mathrm{~F} \quad

Riemann Surfaces
Part II, 2006

Define the branching order vf(p)v_{f}(p) at a point pp and the degree of a non-constant holomorphic map ff between compact Riemann surfaces. State the Riemann-Hurwitz formula.

Let WmC2W_{m} \subset \mathbb{C}^{2} be an affine curve defined by the equation sm=tm+1s^{m}=t^{m}+1, where m2m \geqslant 2 is an integer. Show that the projective curve WˉmP2\bar{W}_{m} \subset \mathbb{P}^{2} corresponding to WmW_{m} is non-singular and identify the points of Wˉm\Wm\bar{W}_{m} \backslash W_{m}. Let FF be a continuous map from Wˉm\bar{W}_{m} to the Riemann sphere S2=C{}S^{2}=\mathbb{C} \cup\{\infty\}, such that the restriction of FF to WmW_{m} is given by F(s,t)=sF(s, t)=s. Show that FF is holomorphic on Wˉm\bar{W}_{m}. Find the degree and the ramification points of FF on Wˉm\bar{W}_{m} and their branching orders. Determine the genus of Wˉm\bar{W}_{m}.

[Basic properties of the complex structure on an algebraic curve may be used without proof if accurately stated.]