Joseph Bertrand

French mathematician who worked in the fields of number theory, differential geometry, probability theory, economics and thermodynamics.

He conjectured, in 1845, that there is at least one prime between n and 2n − 2 for every n > 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.

Bertrand translated into French Carl Friedrich Gauss's work on the theory of errors and the method of least squares.

In the field of economics he reviewed the work on oligopoly theory, specifically the Cournot Competition Model (1838) of French mathematician Antoine Augustin Cournot. His Bertrand Competition Model (1883) argued that Cournot had reached a very misleading conclusion, and he reworked it using prices rather than quantities as the strategic variables, thus showing that the equilibrium price was simply the competitive price.

His book Thermodynamique points out in Chapter XII, that thermodynamic entropy and temperature are only defined for reversible processes. He was one of the first people to point this out.

He died on 5 April 1900, aged 78, in Paris.