Fourier, Jean Baptiste Joseph, Baron, 1768–1830, French mathematician and physicist. He was noted for his researches on heat and on numerical equations. He originated Fourier's theorem on vibratory motion and the Fourier series, which provided a method for representing discontinuous functions by a trigonometric series. Fourier was professor (1795–98) at the École polytechnique, Paris; accompanied Napoleon I to Egypt; and was prefect of Isère (1802–15). In 1808 he was made a baron. He wrote Théorie analytique de la chaleur (1822; tr. 1878, repr. 1955). His scientific writings were collected in two volumes (1888–90).