For every number greater than 1, there is at least one prime number between the number and its double.
Bertrand’s postulate states that there is at least one prime number p, such that n < p < 2n.
Pictorial representation
This Postulate was first proposed by Bertrand in 1845.
However, Bertrand did check that his conjecture was true up to 3 million.
It was first proved in 1850 by Chebyshev so is also called the Bertrand-Chebyshev Theorem.
Chebyshev on a 2021 stamp of Russia
The Indian Mathematician Ramanujan, who was not aware of Chebyshev’s proof, came up with an easier proof on 1919.
In 1932, a Hungarian Mathematician, Paul Erdos, came with another different proof.
