Dedekind cut
He conjectured, in 1845, that there is at least one prime between "n" and 2"n" − 2 for every "n" &gt; 3. Chebyshev proved this conjecture, now called Bertrand's postulate, in 1850. He was also famous for a paradox in the field of probability, now known as Bertrand's Paradox. There is another paradox in game theory that is named after him, called the Bertrand Paradox. In 1849, he was the first to define real numbers using what is now called a Dedekind cut.