* Let $n, k$ are integers number such that $1<n \le k$, does always exist a prime number between $kn$ and $k(n+1)$? 

* When $n=1, k>1$ always exist a prime number between $k$ and $2k$ the question was proved [Bertrand's postulate](https://en.wikipedia.org/wiki/Bertrand%27s_postulate)