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Homework Help: Weierstrass p function

  1. Jun 11, 2009 #1


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    1. The problem statement, all variables and given/known data
    Consider the meromorphic function p' as a function from the torus T to the sphere S. What is its degree? How many ramification points does it have?

    2. Relevant equations
    We have [tex] p'(z)^2 = 4(p(z)-e_1)(p(z)-e_2)(p(z)-e_3)[/tex] where [itex]e_1, e_2, e_3[/itex] are [itex]p(w_1/2), p(w_2/2), p(w_1+w_2/2)[/itex] where we define the torus as an equivalence relation on the complex plane where z~w if z-w is on the lattice generated by [itex]w_1[/itex] and [itex]w_2[/itex]

    3. The attempt at a solution

    p'(z) has a single triple pole at 0 so is of degree 3. Then by Riemann-Hurwitz (X Euler Characteristic), X(T) = 3X(S) -sum of (ramification indices - 1) So sum of (ramification indices - 1) = 6
    We differentiate the above equation to get

    [tex] p''(z)p'(z) = 2p'(z)[(p(z)-e_1)(p(z)-e_2) + (p(z)-e_1)(p(z)-e_3) + (p(z)-e_2)(p(z)-e_3)][/tex]

    Since p(z) has a double pole at 0, p''(z) has a quadruple pole which means 1/p'' has a quadruple zero at z=0. So that takes care of 3 out of the six in the R.H. equation. After that, if p'(z) is non-zero, we get

    [tex] p''(z) = 2[(p(z)-e_1)(p(z)-e_2) + (p(z)-e_1)(p(z)-e_3) + (p(z)-e_2)(p(z)-e_3)][/tex]

    which doesn't offer any immediate suggestions. After this I'm stuck
  2. jcsd
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