Simplify the following text for a Grade 9 native English-speaking person.
Original: 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 .
Simplified: 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 .