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
Post Closed as "Not suitable for this site" by GH from MO, Stanley Yao Xiao, Steven Landsburg, Joonas Ilmavirta, Chris Ramsey