Pierre de Fermat was a 17th Century French lawyer and amateur mathematician. He owned a copy of “Arithmetica” by the Greek mathematician Diophantus, He is mostly known for his work in number theory, especially his statement of 'Fermat's :Last Theorem', (no solution in the integers exists to the equation  
) which he, in a letter, famously claimed to have solved, but which he did not write in his copy of Arithmetica, claiming that, ;the margin is too small to contain it'.
Fermat also contributed to the development of Calculus. While investigating centres of gravity of plane and solid figures, he developed a method for determining maxima, minima and tangents to various curves. Also, using an ingenious trick, he was able to reduce the integral of general power functions to the sums of geometric series.
He also made contributions to probability theory jointly with Pascal, of Pascal's Triangle;e fame, which is used in the binomial expansion.