# Relation between two hypergeometric series

EDIT: Context for this investigation can be found in one of my other MO posts, "Pythagorean Theorem for Right-Corner Hyperbolic Simplices?"

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I'm investigating a function that has led me to this series:

$$T(r) = \sum_{k=0}^{\infty}{2k \choose k}\frac{1}{(k+r)^2 16^k}={}_3F_2(\;\;\frac{1}{2},\;r,\;r\;;\;1+r,\;1+r\;;\;\frac{1}{4}\;\;)\frac{1}{r^2}$$

This paper ("A certain series associated with Catalan's constant") by Victor Adamchik spotlights a tantalizingly similar series ...

$$S(r)=\sum_{k=0}^{\infty}{2k\choose k}^2 \frac{1}{(k+r) \;\; 16^k}={}_3F_2(\;\;\frac{1}{2},\;\frac{1}{2},\;r\;;\;1,\;1+r\;;\;1\;\;)\frac{1}{r}$$

... where the square is on the "wrong" factor.

I'm not very familiar with hypergeometric series (yet). Is there an identity that relates $S$ and $T$?

Edit: I've added the "${}_3F_2$" representations of $S$ and $T$.

Edit2: I have a particular interest in the case $r=1/2$:

$$T(1/2) = 4 \cdot {}_3F_2(\;\;\frac{1}{2},\;\frac{1}{2},\;\frac{1}{2}\;;\;\frac{3}{2},\;\frac{3}{2}\;;\;\frac{1}{4}\;\;) = 4 \cdot \Im( Li_2( \exp( \frac{i \pi}{3} ))) = 4 \;\; \sum_{k=1}^{\infty}\frac{\sin(\frac{\pi}{3}k)}{k^2}$$

$$S(1/2)= 2\cdot {}_3F_2(\;\;\frac{1}{2},\;\frac{1}{2},\;\frac{1}{2}\;;\;1,\;\frac{3}{2}\;;\;1\;\;) = \frac{8G}{\pi}$$

where $G$ is the Catalan constant

$$G = \sum_{k=0}^{\infty}\frac{(-1)^k}{(2k+1)^2} = {}_3 F_2(\;\; \frac{1}{2},\;\frac{1}{2},\;1 \;;\;\frac{3}{2},\;\frac{3}{2}\;;\;-1\;\;)$$

and $Li_2$ is the dilogarithm.

$G$ arises in my analysis of $T$, but the connection isn't clear (to me).

Edit3: Thanks to guidance from the comments, I've expressed $T(1/2)$ in terms of the dilogarithm function. I've also given the "${}_3F_2$" representation of $G$. The connection between $S(1/2)$ (or $G$) and $T(1/2)$ still eludes me, but I've only begun searching through the various caches of identities that have been recommended.

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Mathematica says they're both ${}_3 F_2$ functions. I'll take a look, but in the meantime, have you checked dlmf.nist.gov/16 and functions.wolfram.com/HypergeometricFunctions/Hypergeometric3F2 ? – J. M. Nov 9 '10 at 9:38
@J. M.: Checking them now ... :) – Blue Nov 9 '10 at 10:02
$T(1/2)$ is expressible in terms of dilogarithms: functions.wolfram.com/HypergeometricFunctions/Hypergeometric3F2/… – J. M. Nov 9 '10 at 15:13
From functions.wolfram.com/HypergeometricFunctions/Hypergeometric3F2/… , we get an integral representation for $T(1/2)$: $\int_0^1 \frac1{u}\arcsin\left(\frac{\sqrt{u}}{2}\right)\mathrm{d}u$. – J. M. Nov 9 '10 at 15:20
Another integral representation: $2\int_0^{\pi/6}u\cot\;u\mathrm{d}u$ – J. M. Nov 9 '10 at 15:49