Joseph-Louis Lagrange, comte de l’Empire, original Italian Giuseppe Luigi Lagrangia, (born January 25, 1736, Turin, Sardinia-Piedmont [Italy]—died April 10, 1813, Paris, France), Italian French mathematician who made great contributions to number theory and to analytic and celestial mechanics. His most important book, Mécanique analytique (1788; “Analytic Mechanics”), was the basis for all later work in this field.

Lagrange was from a well-to-do family of French origin on his father’s side. His father was treasurer to the king of Sardinia and lost his fortune in speculation. Lagrange later said, “If I had been rich, I probably would not have devoted myself to mathematics.” His interest in mathematics was aroused by the chance reading of a memoir by the English astronomer Edmond Halley. At 19 (some say 16) he was teaching mathematics at the artillery school of Turin (he would later be instrumental in founding the Turin Academy of Sciences). His early publications, on the propagation of sound and on the concept of maxima and minima (see calculus of variations), were well received; the Swiss mathematician Leonhard Euler praised Lagrange’s version of his theory of variations.