His statement was completely proven by Pafnuty Chebyshev in 1850. For this reason, the postulate is also called the Bertrand-Chebyshev theorem or Chebyshev's theorem. Srinivasa Ramanujan gave a simpler proof. Ramanujan later used that proof when he discovered Ramanujan primes. In 1932, Paul Erdős published a simpler proof using the Chebyshev function θ("x").