Abstract. Is it true that for all integer n > 1 and k ≤ n there exists a
prime number in the interval [kn,(k + 1)n]? The case k = 1 is the Bertrand’s
postulate which was proved for the first time by P. L. Chebyshev in 1850, and
simplified later by P. Erd˝os in 1932, see [2]. The present paper deals with the
case k = 2. A positive answer to the problem for any k ≤ n implies a positive
answer to the old problem whether there is always a prime in the interval
[n2, n2 + n], see [1, p. 11].