# Bertrand's postulate

Bertrand's postulate (actually a theorem) states that for any integer n > 3, there always exists at least one prime number p with n < p < 2n − 2. A weaker but more elegant formulation is: for every 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). Bertrand himself verified his statement for all numbers in the interval [2, 3 × 106]. His conjecture was completely proved by Chebyshev (1821–1894) in 1850 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 $\scriptstyle \pi(x) \,$, where $\scriptstyle \pi(x) \,$ is the prime counting function (number of primes less than or equal to $\scriptstyle x \,$):

$\pi(x) - \pi(\tfrac{x}{2}) \ge 1,\,$for all $\, x \ge 2. \,$

In 1919, Ramanujan (1887–1920) used properties of the Gamma function to give a simpler proof,[1] from which the concept of Ramanujan primes would later arise, and Erdős (1913–1996) in 1932 published a simpler proof using the Chebyshev function ϑ, defined as:

$\vartheta(x) = \sum_{p=2}^{x} \ln (p)$

where px runs over primes, and the binomial coefficients. See proof of Bertrand's postulate for the details.

## Sylvester's theorem

Bertrand's postulate was proposed for applications to permutation groups. 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.

## Erdős's theorems

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).

The prime number theorem (PNT) implies that the number of primes up to x is roughly x/log(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 log(2x) and log(x) are asymptotically equivalent). Therefore the number of primes between n and 2n is roughly n/log(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/log(x2) while the number of primes up to (x + 1)2 is asymptotic to (x + 1)2/log((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.

## Better results

It follows from the prime number theorem that for any real ε > 0, there exists an n0 such that there is always a prime between n and (1 + ε)n for all n > n0: it can be shown, for instance, that

$\lim_{n \to \infty}\frac{\pi((1+\epsilon)n)-\pi(n)}{n/\log n}=\epsilon,$

which implies that π((1 + ε)n) − π(n) goes to infinity (and in particular is greater than 1 for sufficiently large n).[2]

Non-asymptotic bounds have also been proved. In 1952, Jitsuro Nagura proved that for n ≥ 25, there is always a prime between n and (1 + 1/5)n.[3]

In 1976, Lowell Schoenfeld showed that for n ≥ 2010760, there is always a prime between n and (1 + 1/16597)n.[4] In 1998, Pierre Dusart improved the result in his doctoral thesis, showing that for k ≥ 463, pk+1 ≤ (1 + 1/(ln2pk))pk, and in particular for x ≥ 3275, there exists a prime number between x and (1 + 1/(2ln2x))x.[5] In 2010 he proved, that for x ≥ 396738 there is at least one prime between x and (1 + 1/(25ln2x))x.[6]

Generalizations of Bertrand's Postulate have also been obtained by 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.[7] In 2011, Andy Loo proved that there exists a prime between 3n and 4n. Furthermore, he 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 above).[8] None of these proofs require the use of deep analytic results.

## Consequences

• The sequence of primes, along with 1, is a complete sequence; any positive integer can be written as a sum of primes (and 1) using each at most once.
• The number 1 is the only integer which is a harmonic number.