A less restrictive formulation is: for every there is always at least one prime such that
Another formulation, where is the -th prime, is for
This statement was first conjectured in 1845 by Joseph Bertrand (1822–1900). Bertrand himself verified his statement for all numbers in the interval [2, 3 × 106]. His conjecture was completely proved by Chebyshev (1821–1894) in 1852 and so the postulate is also called the Bertrand–Chebyshev theorem or Chebyshev's theorem. Chebyshev's theorem can also be stated as a relationship with , where is the prime counting function (number of primes less than or equal to ):
- , for all .
Prime number theorem
The prime number theorem (PNT) implies that the number of primes up to x is roughly x/ln(x), so if we replace x with 2x then we see the number of primes up to 2x is asymptotically twice the number of primes up to x (the terms ln(2x) and ln(x) are asymptotically equivalent). Therefore the number of primes between n and 2n is roughly n/ln(n) when n is large, and so in particular there are many more primes in this interval than are guaranteed by Bertrand's Postulate. So Bertrand's postulate is comparatively weaker than the PNT. But PNT is a deep theorem, while Bertrand's Postulate can be stated more memorably and proved more easily, and also makes precise claims about what happens for small values of n. (In addition, Chebyshev's theorem was proved before the PNT and so has historical interest.)
The similar and still unsolved Legendre's conjecture asks whether for every n > 1, there is a prime p, such that n2 < p < (n + 1)2. Again we expect that there will be not just one but many primes between n2 and (n + 1)2, but in this case the PNT doesn't help: the number of primes up to x2 is asymptotic to x2/ln(x2) while the number of primes up to (x + 1)2 is asymptotic to (x + 1)2/ln((x + 1)2), which is asymptotic to the estimate on primes up to x2. So unlike the previous case of x and 2x we don't get a proof of Legendre's conjecture even for all large n. Error estimates on the PNT are not (indeed, cannot be) sufficient to prove the existence of even one prime in this interval.
In 1919, Ramanujan (1887–1920) used properties of the Gamma function to give a simpler proof. The short paper included a generalization of the postulate, from which would later arise the concept of Ramanujan primes. Further generalizations of Ramanujan primes have also happened; for instance, there is a proof that
with pk the kth prime and Rn the nth Ramanujan prime.
Other generalizations of Bertrand's Postulate have been obtained using elementary methods. (In the following, n runs through the set of positive integers.) In 2006, M. El Bachraoui proved that there exists a prime between 2n and 3n. In 1973, Denis Hanson proved that there exists a prime between 3n and 4n. Furthermore, in 2011, Andy Loo proved that as n tends to infinity, the number of primes between 3n and 4n also goes to infinity, thereby generalizing Erdős' and Ramanujan's results (see the section on Erdős' theorems below). The first result is obtained with elementary methods. The second one is based on analytic bounds for the factorial function.
Bertrand's postulate was proposed for applications to permutation groups. Sylvester (1814–1897) generalized the weaker statement with the statement: the product of k consecutive integers greater than k is divisible by a prime greater than k. Bertrand's (weaker) postulate follows from this by taking k = n, and considering the k numbers n + 1, n + 2, up to and including n + k = 2n, where n > 1. According to Sylvester's generalization, one of these numbers has a prime factor greater than k. Since all these numbers are less than 2(k + 1), the number with a prime factor greater than k has only one prime factor, and thus is a prime. Note that 2n is not prime, and thus indeed we now know there exists a prime p with n < p < 2n.
where p ≤ x runs over primes. See proof of Bertrand's postulate for the details.
Erdős proved in 1934 that for any positive integer k, there is a natural number N such that for all n > N, there are at least k primes between n and 2n. An equivalent statement had been proved in 1919 by Ramanujan (see Ramanujan prime).
It follows from the prime number theorem that for any real there is a such that for all there is a prime such . It can be shown, for instance, that
which implies that goes to infinity (and, in particular, is greater than 1 for sufficiently large ).
In 1976, Lowell Schoenfeld showed that for , there is always a prime in the open interval .
In his 1998 doctoral thesis, Pierre Dusart improved the above result, showing that for , , and in particular for , there exists a prime in the interval .
In 2016, Pierre Dusart improved his result from 2010, showing (Proposition 5.4) that, if , there is at least one prime in the interval . He also shows (Corollary 5.5) that, for , there is at least one prime in the interval .
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- A proof of the weak version in the Mizar system: http://mizar.org/version/current/html/nat_4.html#T56