1.II.23F

Riemann Surfaces | Part II, 2006

Let Λ=Z+Zτ\Lambda=\mathbb{Z}+\mathbb{Z} \tau be a lattice in C\mathbb{C}, where τ\tau is a fixed complex number with positive imaginary part. The Weierstrass ℘\wp-function is the unique meromorphic Λ\Lambda-periodic function on C\mathbb{C} such that ℘\wp is holomorphic on C\Λ\mathbb{C} \backslash \Lambda, and ℘(z)−1/z2→0\wp(z)-1 / z^{2} \rightarrow 0 as z→0z \rightarrow 0.

Show that ℘(−z)=℘(z)\wp(-z)=\wp(z) and find all the zeros of ℘′\wp^{\prime} in C\mathbb{C}.

Show that ℘\wp satisfies a differential equation

℘′(z)2=Q(℘(z))\wp^{\prime}(z)^{2}=Q(\wp(z))

for some cubic polynomial Q(w)Q(w). Further show that

Q(w)=4(w−℘(12))(w−℘(12τ))(w−℘(12(1+τ)))Q(w)=4\left(w-\wp\left(\frac{1}{2}\right)\right)\left(w-\wp\left(\frac{1}{2} \tau\right)\right)\left(w-\wp\left(\frac{1}{2}(1+\tau)\right)\right)

and that the three roots of QQ are distinct.

[Standard properties of meromorphic doubly-periodic functions may be used without proof provided these are accurately stated, but any properties of the ℘\wp-function that you use must be deduced from first principles.]

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