when $\int x^m (a+bx^n)^p dx$ is elementary function

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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?

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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".