Bertrand's postulate states that if n is a positive integer, then for n > 3 there always exists at least one prime number p between n and 2n-2, or in an equivalent weaker but more elegant form then for n > 1 there is always at least one prime p such that n < p < 2n.
This statement was first conjectured in 1845 by Joseph Bertrand[?] (1822-1900). His conjecture was completely proved by Pafnuty Lvovich Chebyshev (1821-1894) in 1850 and so the postulate is also called Chebyshev's theorem. Chebyshev in his proof used the Chebyshev's inequality. Bertrand himself verified his statement for all numbers in the interval [2, 3 × 106].
Srinivasa Aaiyangar Ramanujan (1887-1920) gave a simpler proof and Paul Erdös (1913-1996) in 1932 published a very simple proof where he used the function θ(x), defined as:
where p ≤ x runs over primes, and the binomial coefficients.
Bertrand's postulate was proposed for applications to permutation groups. James Joseph Sylvester (1814-1897) generalized it with the statement: the product of k consecutive integers greater than k is divisible by a prime greater than k.
A similar and still unsolved conjecture is asking for a prime p, such that n2 < p < (n+1)2.
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