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# Simeon Denis Poisson

**Siméon-Denis Poisson** (Pithiviers, June 21, 1781 – Sceaux, April 25, 1840) was a French mathematician known for his work on definite integrals, electromagnetic theory and probability theory.

Poisson started a study of medicine on advice of his parents, but soon abandoned this study in favor of mathematics. In 1798 he entered the École Polytechnique where among his teachers were the mathematicians Laplace and Lagrange, whom he befriended for life. Until his death Poisson was almost entirely engaged in mathematical research and in teaching. He became an associate professor at the École Polytechnique in 1802 and a full professor in 1806. In 1808 he was made astronomer at the Bureau des Longitudes, and, when the Faculté des Sciences was instituted in 1809, he was appointed professor of pure mathematics.

Poisson's most important work concerned the application of mathematics to electricity and magnetism, potential theory, and other parts of physics. His *Traité de mécanique* (Treatise of mechanics) (1811, 1833) was the standard work on mechanics during the first half of the 19th century. In 1812 he published a work that contained many of the most useful laws of electrostatics, as well as his theory that electricity is made up of two fluids.

In pure mathematics his most important papers were a series of publications on definite integrals and Fourier series. This latter work paved the way for Peter Dirichlet and Bernhard Riemann on the same subject.

In *Recherches sur la probabilité des jugements en matière criminelle et en matière civile* (Research on the Probability of Criminal and Civil Verdicts) (1837), an important work on probability theory, the Poisson distribution first appeared. Poisson's other works include *Théorie nouvelle de l'action capillaire* (A new theory of capillary action) (1831) and *Théorie mathematiques de la chaleur (Mathematical theory of heat) (1835).*

Poisson's integral, Poisson's equation in potential theory , Poisson's brackets in differential geometry, and Poisson's ratio in elasticity are indicative of the scope and importance of his researches.