Thomas Fantet de Lagny

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Born: 7 Nov 1660 in Lyon, France
Died: 11 April 1734 in Paris, France

Thomas De Lagny was taught first by his uncle, then he entered a Jesuit college in Lyon. After this he studied law in Toulouse.

In 1686 De Lagny became a mathematics tutor to a family in Paris, a position he held for about 10 years. He collaborated with de L'Hôpital while in Paris. In 1695 he became a member of the Académie Royale des Sciences, then two years later he became professor of hydrography at Rochefort. He also held positions as librarian at Bibliothéque du Roi for a time and spent two years as deputy director of a bank.

De Lagny is well known for his contributions to computational mathematics, calculating p to 120 places and also making useful comments on the convergence of the series he was using.

He constructed trigonometric tables and used binary arithmetic. In 1733 de Lagny examined the continued fraction expansion of the quotient of two integers and, as an example, considered adjacent Fibonacci numbers as the worst case expansion for the Euclidean algorithm.