Thomas Fantet de Lagny (7 November 1660 – 11 April 1734) was a French

mathematician A mathematician is someone who uses an extensive knowledge of mathematics in their work, typically to solve mathematical problems. Mathematicians are concerned with numbers, data, quantity, mathematical structure, structure, space, Mathematica ...

, well known for his contributions to computational mathematics, and for calculating π to 112 correct decimal places.

Biography

Thomas Fantet de Lagny was son of Pierre Fantet, a royal official in

Grenoble Grenoble ( ; ; or ; or ) is the Prefectures in France, prefecture and List of communes in France with over 20,000 inhabitants, largest city of the Isère Departments of France, department in the Auvergne-Rhône-Alpes Regions of France, region ...

, and Jeanne d'Azy, the daughter of a physician from

Montpellier Montpellier (; ) is a city in southern France near the Mediterranean Sea. One of the largest urban centres in the region of Occitania (administrative region), Occitania, Montpellier is the prefecture of the Departments of France, department of ...

. He entered a Jesuit College in

Lyon Lyon (Franco-Provençal: ''Liyon'') is a city in France. It is located at the confluence of the rivers Rhône and Saône, to the northwest of the French Alps, southeast of Paris, north of Marseille, southwest of Geneva, Switzerland, north ...

, where he became passionate about

mathematics Mathematics is a field of study that discovers and organizes methods, Mathematical theory, theories and theorems that are developed and Mathematical proof, proved for the needs of empirical sciences and mathematics itself. There are many ar ...

, as he studied some mathematical texts such as ''Euclid'' by Georges Fournier and an

algebra Algebra is a branch of mathematics that deals with abstract systems, known as algebraic structures, and the manipulation of expressions within those systems. It is a generalization of arithmetic that introduces variables and algebraic ope ...

text by Jacques Pelletier du Mans. Then he studied three years in the Faculty of Law in

Toulouse Toulouse (, ; ; ) is a city in southern France, the Prefectures in France, prefecture of the Haute-Garonne department and of the Occitania (administrative region), Occitania region. The city is on the banks of the Garonne, River Garonne, from ...

. In 1686, he went to

Paris Paris () is the Capital city, capital and List of communes in France with over 20,000 inhabitants, largest city of France. With an estimated population of 2,048,472 residents in January 2025 in an area of more than , Paris is the List of ci ...

and became a mathematics tutor to the Noailles family. He collaborated with de l'Hospital under the name of ''de Lagny'', and at that time he started publishing his first mathematical papers.
He came back to

when, on 11 December 1695, he was named an associate of the

Académie Royale des Sciences The French Academy of Sciences (, ) is a learned society, founded in 1666 by Louis XIV at the suggestion of Jean-Baptiste Colbert, to encourage and protect the spirit of French scientific research. It was at the forefront of scientific d ...

. Then, in 1697, he became professor of

hydrography Hydrography is the branch of applied sciences which deals with the measurement and description of the physical features of oceans, seas, coastal areas, lakes and rivers, as well as with the prediction of their change over time, for the primary ...

at Rochefort for 16 years. De Lagny returned to

in 1714, and became a librarian at the ''Bibliothèque du roi'', and a deputy director of the ''Banque Générale'' between 1716 and 1718. On 7 July 1719, he was awarded a pension by the

, finally earning his living from science. In 1723, he became a pensionnaire at the academy, replacing

Pierre Varignon Pierre Varignon (; 1654 – 23 December 1722) was a French mathematician. He was educated at the Society of Jesus, Jesuit College and the University of Caen, where he received his Magister Artium, M.A. in 1682. He took Holy Orders the following ...

who died in 1722, but had to retire in 1733. De Lagny died on 11 April 1734. While he was dying, someone asked him: "What is the square of 12?" and he answered immediately: "144."

Computing π

In 1719, de Lagny calculated π to 127 decimal places, using Gregory's series for arctangent, but only 112 decimals were correct. This remained the record until 1789, when Jurij Vega calculated 126 correct digits of π.

Bibliography

* ''Méthode nouvelle infiniment générale et infiniment abrégée pour l’extraction des racines quarrées, cubiques...'' (Paris, 1691) * ''Méthodes nouvelles et abrégées pour l’extraction et l’approximation des racines'' (Paris, 1692) * ''Nouveaux élémens d’arithmétique et d’algébre ou introduction aux mathématiques'' (Paris, 1697) * ''Trignonmétrie française ou reformée'' (Rochefort, 1703) * ''De la cubature de la sphére où l’on démontr une infinité de portions de sphére égales à des pyramides rectilignes'' (La Rochelle, 1705) * ''Analyse générale ou Méthodes nouvelles pour résoudre les probémes de tous les genres et de tous degrés à l’infini'', M. Richer, ed. (Paris, 1733)

References

*
Lagny, Thomas Fantet de, Encyclopedia.com
{{DEFAULTSORT:Lagny, Thomas Fantet de 1660 births 1734 deaths French mathematicians Pi-related people