In one of the answers of the question Integration of sqrt Sin x dx, I saw something similiar to that:
$m,n,p \neq 0 \in \mathbb{Q}$
$\int x^m (a+bx^n)^p dx$ is elementary function $\implies$ $\frac{m+1}{n} \in \mathbb{Z}$ or $\frac{m+1}{n} + p \in \mathbb{Z}$
is it true?
the other direction also true?
and where can I find a proof of this proposition?
This is a result of Chebyshev.
A Google search for "chebychev elementary functions integration" turns up a number of relevant hits including these:
http://www2.onu.edu/~m-caragiu.1/bonus_files/CHEBYSHE.pdf
http://www.rangevoting.org/MarchisottoZint.pdf
Also look up "integration in finite terms".