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I was solving an integral I made up (not included because I would prefer if the answer was not just a different way of solving the initial integral) as a fun challenge (as one does) and my solution contained the following summation: $$\sum_{n=1}^{\infty}\sum_{k=1}^{\infty}\frac{\left(-1\right)^{n+k}}{\alpha n^{2}k+\beta nk^{2}} \text{ where }\alpha,\beta>0.$$ I found its symmetry very interesting and could find nothing about it or any similar double summations online. My only idea involves using the Lerch Transcendent to solve one summation to get: $$\sum_{k=1}^{\infty}\left(-1\right)^{k}\frac{\Phi\left(-1,1,\frac{k\beta}{\alpha}+1\right)-\ln2}{\beta k^{2}}$$

$$=\frac{\pi^{2}\ln2}{12\beta}+\sum_{k=1}^\infty\frac{\left(-1\right)^{k}\Phi\left(-1,1,\frac{k\beta}{\alpha}+1\right)}{\beta k^{2}}$$

$$=\frac{\pi^{2}\ln2}{12\beta}+-\frac{1}{\beta}\sum_{k=1}^{\infty}\frac{\left(-1\right)^{k}\Phi\left(-1,1,\frac{k\beta}{\alpha}\right)}{k^{2}}+\frac{3}{4}\frac{\beta}{\alpha^{2}}\zeta\left(3\right)$$

$$=-\frac{1}{\beta}\sum_{k=1}^{30}\frac{\left(-1\right)^{k}}{k^{2}}\left(\psi\left(\frac{k\beta}{\alpha}\right)-\psi\left(\frac{k\beta}{2\alpha}\right)\right)+\frac{3}{4}\frac{\beta}{\alpha^{2}}\zeta\left(3\right)$$

None of these seem to be helpful. Does this sum have a closed form?

Dylan Levine
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    Lattice sums of this type can only be evaluated in closed form for very special values of $\alpha$, $\beta$ and require very advanced concepts from class field theory or the Kronecker limit formulae to do so. You will not be able to obtain a general closed form through elementary means. – KStar Apr 07 '25 at 20:39
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    To see examples of the relevant machinery, see 1, 2, 3, 4. I will leave you to look into say Herglotz-type integrals for further examples. – KStar Apr 07 '25 at 20:46
  • Where is $30$ coming from ? – Claude Leibovici Apr 08 '25 at 04:58
  • Simpler: $$-\frac{3 \beta \zeta (3)}{4 \alpha ^2}+\int_0^1 \frac{\log (1+x) \log \left(1+x^{-\frac{\beta }{\alpha }}\right)}{x \alpha } , dx$$ – Mariusz Iwaniuk Apr 08 '25 at 10:11
  • @MariuszIwaniuk Looks like you pretty much figured out my original integral :) – Dylan Levine Apr 08 '25 at 13:36

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