$n$-th derivative of Beta function

921 Views Asked by At

We pretty much know nothing about the high order derivatives of the Beta function. Well, we known for the example some recursive formulae for $\Gamma^{(n)}(1)$ as well as $\Gamma^{(n)}\left(\frac{1}{2}\right)$ but that is all. Yesterday , I encountered the following integral:

$$\int_{0}^{\infty} \frac{\log^n x}{\left ( 1+x^2 \right )^m}\, {\rm d}x$$

As it can easily be seen , we have that:

$$\int_{0}^{\infty} \frac{\log x}{\left ( 1+x^2 \right )^m}\, {\rm d}x ={\rm B}^{(1)} \left ( \frac{1}{2}, m- \frac{1}{2} \right ) \implies \int_{0}^{\infty} \frac{\log^n x}{\left ( 1+x^2 \right )^m}\, {\rm d}x = {\rm B}^{(n)} \left ( \frac{1}{2}, m- \frac{1}{2} \right )$$

The question is whether or not we can express that last $n$-th derivative in a closed form - most likely in an inductive form. I guess it will be difficult but with mathematics you never know.

1

There are 1 best solutions below

0
On BEST ANSWER

Let us denote with

$$f(n, m)=\int_0^\infty \frac{\log^n x}{(1+x^2)^m}\, {\rm d}x$$

It is quite straigtforward to note that:

$$f(n, 1)=\int_{0}^{\infty} \frac{\log^n x}{1+x^2}\, {\rm d}x = \left\{\begin{matrix} 0 &, &{\rm odd} \\ 2n! \beta(n+1)&, &{\rm even} \end{matrix}\right.$$

Here $\beta$ denotes the Beta dirichlet function.

On the other hand

$$f(1,m)= \int_{0}^{\infty}\frac{\log x}{\left ( 1+x^2 \right )^m}= {\rm B}^{(1)} \left ( \frac{1}{2}, m - \frac{1}{2} \right )= \frac{\Gamma \left ( \frac{1}{2} \right ) \Gamma \left ( m- \frac{1}{2} \right ) \bigg[ \psi^{(0)}\left ( \frac{1}{2}\right) - \psi^{(0)} \left ( m- \frac{1}{2} \right ) \bigg]}{m^2 \Gamma(m)}$$

Using gammatester's comment for the more general result, that is for $f(n,m)$ using Liebniz's general rule we have that:

$$f(n, m)= \int_{0}^{\infty} \frac{\log^n x}{\left ( 1+x^2 \right )^m}\, {\rm d}x = {\rm B}^{(n)} \left ( \frac{1}{2}, m- \frac{1}{2} \right )= \frac{1}{m^2 \Gamma(m)}\sum_{k=0}^{n} \binom{n}{k} \Gamma^{(k)} \left ( \frac{1}{2} \right )\Gamma^{(n-k)} \left ( m- \frac{1}{2} \right )$$

Of course for $\Gamma^{(n)} \left(\frac{1}{2} \right)$ we have the following reduction formula:

$$\Gamma^{(n+1)}\left ( \frac{1}{2} \right )= -\left ( \gamma +2 \log 2 \right ) \Gamma^{(n)} \left ( \frac{1}{2} \right ) +n ! \sum_{k=1}^{n}\frac{(-1)^{k+1}}{\left ( n-k \right )!} \left ( 2^{k+1}-1 \right )\zeta(k+1) \Gamma^{(n-k)}\left ( \frac{1}{2} \right )$$