Paper 3, Section II, I

Riemann Surfaces
Part II, 2012

Let Λ\Lambda be the lattice Z+Zi,X\mathbb{Z}+\mathbb{Z} i, X the torus C/Λ\mathbb{C} / \Lambda, and \wp the Weierstrass elliptic function with respect to Λ\Lambda.

(i) Let xXx \in X be the point given by 0Λ0 \in \Lambda. Determine the group

G={fAut(X)f(x)=x}G=\{f \in \operatorname{Aut}(X) \mid f(x)=x\}

(ii) Show that 2\wp^{2} defines a degree 4 holomorphic map h:XC{}h: X \rightarrow \mathbb{C} \cup\{\infty\}, which is invariant under the action of GG, that is, h(f(y))=h(y)h(f(y))=h(y) for any yXy \in X and any fGf \in G. Identify a ramification point of hh distinct from xx which is fixed by every element of GG.

[If you use the Monodromy theorem, then you should state it correctly. You may use the fact that Aut(C)={az+baC\{0},bC}\operatorname{Aut}(\mathbb{C})=\{a z+b \mid a \in \mathbb{C} \backslash\{0\}, b \in \mathbb{C}\}, and may assume without proof standard facts about \wp.]