# Jean le Rond d'Alembert

**Jean le Rond d'Alembert** (November 16, 1717 – October 29, 1783) was a French mathematician, mechanician, physicist and philosopher. He was also one of the editors of the *Encyclopédie*, an early French encyclopedia. D'Alembert's method for the wave equation is named after him.

## Childhood

Born d'Alembert was the illegitimate child of the writer Claudine Guérin de Tencin and the chevalier Louis-Camus Destouches (an artillery officer). Destouches was abroad at the time of d'Alembert's birth, and a couple of days after birth his mother left him on the steps of the Saint-Jean-le-Rond de Paris church. According to custom, he is named after the protecting saint of the church. d'Alembert was placed in an orphanage but was soon adopted by the wife of a glazier. Destouches secretly paid for the education of Jean le Rond, but did not want his parentage officially recognised.

## Studies

D'Alembert first attended a private school. The chevalier Destouches left d'Alembert an annuity of 1200 livres on his death in 1726. Under the influence of the Destouches family, at the age of twelve d'Alembert entered the jansenist Collège des Quatre-Nations (the institution was also known under the name "Collège Mazarin"). Here he studied philosophy, law, and art, graduating as *bachelier* in 1735. In his later life, d'Alembert scorned the Cartesian principles he had been taught by the Jansenists: "physical premotion, innate ideas and the vortices".

The Jansenists steered d'Alembert toward an ecclesiastical career, attempting to deter him from pursuits such as poetry and mathematics. Theology was, however, "rather unsubstantial fodder" for d'Alembert. He entered law school for two years, and was nominated *avocat* in 1738.

He was also interested in medicine and mathematics. Jean le Rond was first registered under the name *Daremberg*, but later changed it to *d'Alembert*. In July of 1739 he made his first contribution to the field of mathematics, pointing out the errors he had detected in *L'analyse démontrée* (published 1708 by Charles René Reynaud) in a communication addressed to the Académie des Sciences. At the time *L'analyse démontrée* was a standard work, which d'Alembert himself had used to study the foundations of mathematics. D'Alembert was also a Latin scholar of some note and worked in the latter part of his life on a superb translation of Tacitus, from which he received wide praise including that of Diderot.

In 1740, he submitted his second scientific work from the field of fluid mechanics *Memoire sur le refraction des corps solides*, which was recognized by Clairaut. In this work d'Alembert theoretically explained refraction. He also wrote about what is now called D'Alembert's paradox: that the drag on a body immersed in an inviscid, incompressible fluid is zero.

While he made great strides in mathematics and physics, d'Alembert is also famously known for incorrectly arguing in *Croix ou Pile* that the probability of a coin landing heads increased for every time that it came up tails. In gambling, the strategy of decreasing one's bet the more one wins and increasing one's bet the more one loses is therefore called *the d'Alembert system*, a type of martingale.

He suffered bad health for many years and his death was as the result of a bladder illness. As a known unbeliever, d'Alembert was buried in a common unmarked grave.

In France, the fundamental theorem of algebra is known as the d'Alembert/Gauss theorem.

He also created his ratio test, a test to see if a series converges.

See also D'Alembert's principle.

## Bibliography

*Jean d'Alembert*by Ronald Grimsley. (1963)