**Alexis Claude Clairault** (or **Clairaut**) (May 3, 1713 - May 17, 1765) was a French mathematician.

He was born in Paris, France, where his father taught mathematics. Under his father's tuition he made such rapid progress in the subject that in his thirteenth year he read before the Académie française an account of the properties of four curves which he had then discovered. When only sixteen he finished a treatise, *Recherches sur les courbes a double courbure*, which, on its publication in 1731, procured his admission into the Academy of Sciences, although he was below the legal age.

In 1736, together with Pierre Louis Maupertuis, he took part in the expedition to Lapland, which was undertaken for the purpose of estimating a degree of the meridian, and on his return he published his treatise *Théorie de la figure de la terre* (1743). In this work he promulgated the theorem, known as "Clairault's theorem," which connects the gravity at points on the surface of a rotating ellipsoid with the compression and the centrifugal force at the equator.

He obtained an ingenious approximate solution of the problem of the three bodies; in 1750 he gained the prize of the St Petersburg Academy for his essay *Théorie de la lune*; and in 1759 he calculated the perihelion of Halley's comet. He also detected singular solutions in differential equations of the first order, and of the second and higher degrees. Clairault died at Paris.

*This entry was originally from the 1911 Encyclopedia Britannica.*