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The Weierstraß $\wp$ function for a lattice $\Lambda\subset\Bbb C$ can be defined by the sum

$$ \wp(z) = \frac1{z^2} ~+\!\! \sum_{\lambda\in\Lambda\setminus\{0\}} \left(\frac1{(z-\lambda)^2}-\frac1{\lambda^2}\right) \tag1 $$ and there is the following representation of $\csc$: $$ f(x) := \pi^2\csc^2(\pi x) = \sum_{n\in\Bbb Z}\frac1{(x+n)^2} \tag 2 $$

My question is if (and how) $\wp$ can be expressed as a "one-dimensional" sum over $f(x)$ or other trigonometric functions. For simplicity, let's assume that $\Lambda=\Bbb Z + \Bbb Zi$.

Splitting the sum $(1)$ into two sums, $A$ over $\lambda\in\Bbb R$, and $B$ over $\lambda\in\Bbb C\setminus \Bbb R$:

$$ \wp(z) = \underbrace{\frac1{z^2} ~+\!\! \sum_{n\in\Bbb Z\setminus\{0\}} \left(\frac1{(z-n)^2}-\frac1{n^2}\right)}_{\textstyle=:A(z)} ~+\!\!\underbrace{\sum_{m\in\Bbb Z\setminus\{0\}} \sum_{n\in\Bbb Z} \left(\frac1{(z-n-mi)^2}-\frac1{(n+mi)^2}\right)}_{\textstyle=:B(z)} \tag 3 $$ This should be correct because $(1)$ converges absolutely and thus the series can be rearranged. The two summands of $A$ also converge absolutely, and therefore we have:

$$ A(z) = f(z) - 2\zeta(2) \tag 4 $$ Sum $B$ is very similar, but there's an additional sum in the imaginary direction over $f$:

$$\begin{align} B(z) &=\sum_{m\in\Bbb Z\setminus\{0\}} \left(f(z-mi) -\sum_{n\in\Bbb Z} \frac1{(n+mi)^2}\right) \tag{5.1}\\ &=\sum_{m\in\Bbb Z\setminus\{0\}} \bigl(f(z-mi) - f(mi)\bigr) \tag{5.2}\\ &=\sum_{m\in\Bbb Z\setminus\{0\}} f(z-mi) - \underbrace{\sum_{m\in\Bbb Z\setminus\{0\}} f(mi)}_{\textstyle =:C} \tag{5.3} \end{align}$$

Taking it all together, it's $$ \wp(z) =A(z)+B(z) = -2\zeta(2) -C +\sum_{m\in\Bbb Z} f(z-mi) \tag 6 $$

But then I am stuck to determine $C$.

emacs drives me nuts
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1 Answers1

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$$\begin{align} \sum_{n\in\Bbb Z\setminus\{0\}} \csc^2(n\pi i) ~&= \sum_{n\in\Bbb Z\setminus\{0\}} \frac1{\sin^2(n\pi i)}\\ ~&\stackrel{(10)}= -\!\!\sum_{n\in\Bbb Z\setminus\{0\}} \frac1{\sinh^2(n\pi)}\\ ~&\stackrel{(11)}= -2\sum_{n=1}^\infty \frac1{\sinh^2(n\pi)}\\ ~&\stackrel{(12)}= -2\left(\frac16-\frac1{2\pi}\right)\\ \end{align}$$ where $(10)$ uses $\sin(iz)=i\sinh(z)$, $(11)$ uses that $\sinh^2$ is an even function, and $(12)$ is according to this thread.

So the value for $C$ is: $$\begin{align} C &= \sum_{m\in\Bbb Z\setminus\{0\}} f(mi)\\ &= \pi^2 \!\!\sum_{n\in\Bbb Z\setminus\{0\}} \csc^2(n\pi i)\\ &= -2\frac{\pi^2}6 +\pi\\ \end{align}$$ Finally, $$\begin{align} \wp_{\Bbb Z+\Bbb Zi}(z)&=-2\zeta(2)-C+\!\!\sum_{m\in\Bbb Z\setminus\{0\}} f(z-mi) \\ &=-\pi+\pi^2\!\!\!\sum_{m\in\Bbb Z\setminus\{0\}} \frac1{\sin^2(\pi(z-mi)))} \\ \end{align}$$

emacs drives me nuts
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