Hypergeometric Function and it’s relation to Gamma function

gamma functionhypergeometric functionindefinite-integralsintegration

I've been trying to solve a Quantum mechanics problem, where I have to do the integration below:
$$
\int_{-\infty}^{+\infty} \frac1{(x^2+b^2)^{2n}}dx
$$

with $b$ and $n$ not specified (the problem wants to find the general equation. I used Mathematica for the indefinite
$$
\frac13 x^3 (c^2 + x^2)^k (1 + \frac{x^2}{c^2})^{-k} (Hypergeometric2F1\left[\frac32, -k,\frac52, -\left(\frac{x^2}{c^2}\right)\right]
$$

Now I'm not familiar with Hypergeometric function so I looked up the solution and found out it solved it using
$$
\int_0^\infty\frac{x^k}{(x^2+b^2)^{\ell}}dx = \frac1{2b^{2\ell-k-1}}\frac{\Gamma(\frac{k+1}2)\Gamma(\frac{2\ell-k-1}2)}{\Gamma(\ell)}$$

Since the function is symmetric the first integration is eqaul to twice of the second integration with $\ell = 2n$ and $k=0$.

Can someone explain how can we derive from the first integration the second one? or is there any relation between hypergeometric and gamma functions.

Best Answer

$\newcommand{\d}{\mathrm{d}}\newcommand{\B}{\mathfrak{B}}$We don't need hypergeometric functions here. $$\begin{align}I_n:&=\int_{\Bbb R}\frac{1}{(x^2+b^2)^{2n}}\,\d x\\&=\frac{2}{b^{4n}}\int_0^\infty\frac{1}{(x^2/b^2+1)^{2n}}\,\d x\\&=\frac{2}{b^{4n-1}}\int_0^\infty\frac{1}{(x^2+1)^{2n}}\,\d x\\&\overset{u=x^2+1}{=}\frac{1}{b^{4n-1}}\int_1^\infty u^{-2n}(u-1)^{-1/2}\,\d u\\&\overset{t=1/u}{=}\frac{1}{b^{4n-1}}\int_0^1 t^{2n-2}\left(\frac{1}{t}-1\right)^{-1/2}\,\d t\\&=\frac{1}{b^{4n-1}}\int_0^1t^{(2n-1/2)-1}(1-t)^{(1/2)-1}\,\d t\\&=\frac{1}{b^{4n-1}}\B\left(2n-\frac{1}{2},\,\frac{1}{2}\right)\\&=\frac{1}{b^{4n-1}}\cdot\frac{\Gamma\left(2n-\frac{1}{2}\right)\Gamma\left(\frac{1}{2}\right)}{(2n-1)!}\\&=\frac{\sqrt{\pi}}{b^{4n-1}}\cdot\frac{1}{(2n-1)!}\cdot(2n-1-1/2)(2n-2-1/2)\cdots(1-1/2)\Gamma(1/2)\\&=\frac{\pi}{b^{4n-1}\cdot 2^{2n-1}}\cdot\frac{(4n-3)!!}{(2n-1)!}\end{align}$$

Where appeared $\B$ the Beta function and $\Gamma$ the gamma function, with which you clearly already are familiar. I also used $\Gamma(1/2)=\sqrt{\pi}$.