# Thread: n-fold integral

1. ## n-fold integral

Can anyone give me any suggestion of how to solve this integral? Or better - has anyone seen any solution to this kind of integral?

$
\int_0^1\int_0^1\cdots \int_0^1\frac{1}{\left( c+x_{1}+x_{2}+\cdots +x_{n}\right) }dx_{1}\cdots dx_{n}
$

I would be grateful for any help!

2. Originally Posted by Per
Can anyone give me any suggestion of how to solve this integral? Or better - has anyone seen any solution to this kind of integral?

$
\int_0^1\int_0^1\cdots \int_0^1\frac{1}{\left( c+x_{1}+x_{2}+\cdots +x_{n}\right) }dx_{1}\cdots dx_{n}
$

I would be grateful for any help!

Assuming $c+x_1+\ldots+x_n>0\,,\,\,\forall \,0\leq x_i\leq 1$ :

Beginning with the first one: $\int^1_0\frac{1}{c+x_1+\ldots +x_n}\,dx_n=[\ln(c+x_1+\ldots +x_n)]^1_0=$ $\ln(c+x_1+\ldots +x_{n-1}+1)-\ln(c+x_1+\ldots +x_{n-1})$ ... I purposedly

didn't write this as the logarithm of a quotient since for the next step is easier to integrate this way.

Next step: $\int^1_0\left(\ln(c+x_1+\ldots +x_{n-1}+1)-\ln(c+x_1+\ldots +x_{n-1})\right)\,dx_{n-1}=$ $\left[(c+x_1+\ldots +x_{n-1}+1)(\ln(c+x_1+\ldots +x_{n-1}+1)-1)\right]^1_0$

$-\left[(c+x_1+\ldots +x_{n-1})(\ln(c+x_1+\ldots +x_{n-1})-1)\right]^1_0=$ ...etc.

The above doesn't look too hard but it looks pretty messy and ugly...I don't know if there's a nice expression for it all.

Tonio