Solution

ID: past-exam-of-the-mathematics-course-of-the-university-of-cambridge/2009/ii/paper-2/23g/a/solution

An elliptic function for a lattice is a meromorphic function on satisfying for all . Its order is the number of poles, counted with multiplicity, in a fundamental parallelogram, equivalently the degree of its map from the torus to the Riemann sphere. Differentiation preserves the periods. If the distinct poles have orders , the derivative has exactly the same pole locations with orders . Therefore
The bounds use , valid for a nonconstant elliptic function.
For the Weierstrass elliptic function , has a single pole of order five, so its derivative has order six. For the second example choose not a critical value of and choose away from zero and the two points where . Then
has a double pole at and three simple poles: the quotient has residue at zero and residue 1 at each of its two other poles. No locations overlap. Hence its order is five and its derivative has order , as required.

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