1

$$\sum_{n=1}^{\infty}\frac{\left(-1\right)^{n}}{n}\left(\frac{e^{-\pi n}}{1+e^{-\pi n}}\right)=\frac{\pi-5\ln2}{8} \tag{1}$$

I am searching for an elementary way to prove this considering the simplicity of the result.

This is how I basically came up with it.

Consider the Function: $$\delta(x)=\sum_{n=1}^{\infty}\frac{1}{n}\left(\frac{x^n}{1-x^n}\right)$$

Now it can be shown that: $$-2\delta\left(x^{4}\right)+3\delta\left(x^{2}\right)-\delta\left(x\right)=\sum_{n=1}^{\infty}\frac{\left(-1\right)^{n}}{n}\left(\frac{x^{n}}{1+x^{n}}\right) \tag{2}$$

What's interesting here is that we have Closed Form Values of the Function Evaluated at $\left(e^{-\pi}\right)^1$, $\left(e^{-\pi}\right)^2$, $\left(e^{-\pi}\right)^4$ which were given by Sir Ramanujan as far as I know.
These are,

$$\delta(e^{-\pi})=\frac{7}{8}\ln2+\frac{3}{4}\ln\pi-\frac{1}{24}\pi-\ln\Gamma\left(\frac{1}{4}\right)$$ $$\delta(e^{-2\pi})=\ln2+\frac{3}{4}\ln\pi-\frac{1}{12}\pi-\ln\Gamma\left(\frac{1}{4}\right)$$ $$\delta(e^{-4\pi})=\frac{11}{8}\ln2+\frac{3}{4}\ln\pi-\frac{1}{6}\pi-\ln\Gamma\left(\frac{1}{4}\right)$$

Now putting $e^{-\pi}$ in $(2)$ Results in $(1)$.

Is there an elementary way to prove this considering the Simplicity of the Result.

Miracle Invoker
  • 3,200
  • 1
  • 3
  • 22
  • The sums in question are related to the sums given by Plouffe. See in particular https://math.stackexchange.com/q/938123/72031 – Paramanand Singh Oct 07 '23 at 10:36
  • @ParamanandSingh Thank you for the comment, as I can see these are deeply related to elliptic integrals so I presume there might be no elementary way. – Miracle Invoker Oct 07 '23 at 10:55
  • It's not too hard to check that it's equivalent to show: $$\prod_{n\ge0}\frac{1+e^{-2\pi n}}{1+e^{-(2n+1)\pi}}\overset{??}{=}e^{\pi/8}\cdot2^{3/8}$$ – FShrike Oct 07 '23 at 13:45

0 Answers0