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 . His conjecture was completely proved by Chebyshev ( 1821 â `` 1894 ) in 1850 and so the postulate is also called the Bertrand-Chebyshev theorem or Chebyshev 's theorem .