Pierre de Fermat was a French mathematician and Lawyer. He made contributions that led to infinitesimal calculus, as well as number theory, probability, and analytic geometry. He is best known for a number theory result known as Fermat's Last Theorem. He described the result in the margin of Diophantus' Arithmetica claiming to have a proof that was too large for the margin of the book. It was over 350 years later that a generally accepted mathematical proof was found, it was over 100 pages long and contained mathematics not known in during Fermat's lifetime.
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Stamp  Description 

Czechoslovakia, 7k, 2000 Scott: 3122 A1177 

France, 4.50fr, 2001 Scott: 2836 A1575 