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, structure, space, models, and change. History On ...

, 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 lat, Gratianopolis , commune status = Prefecture and commune , image = Panorama grenoble.png , image size = , caption = From upper left: Panorama of the city, Grenoble’s cable cars, place Saint- ...

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

Montpellier Montpellier (, , ; oc, Montpelhièr ) 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 ...

. He entered a Jesuit College in

Lyon Lyon,, ; Occitan: ''Lion'', hist. ''Lionés'' also spelled in English as Lyons, is the third-largest city and second-largest metropolitan area of France. It is located at the confluence of the rivers Rhône and Saône, to the northwest of t ...

, where he became passionate about

mathematics Mathematics is an area of knowledge that includes the topics of numbers, formulas and related structures, shapes and the spaces in which they are contained, and quantities and their changes. These topics are represented in modern mathematics ...

, as he studied some mathematical texts such as ''Euclid'' by

Georges Fournier Georges Fournier (21 November 1881 – 1 December 1954) was a French astronomer.Camille Flammarion, G.C. Flammarion, J. Camus''Georges Fournier.''L'Astronomie, Vol. 69 (1955), p. 201 He observed the planet Mars with great detail. In 1909 he ...

and an

algebra Algebra () is one of the broad areas of mathematics. Roughly speaking, algebra is the study of mathematical symbols and the rules for manipulating these symbols in formulas; it is a unifying thread of almost all of mathematics. Elementary a ...

text by

Jacques Pelletier du Mans Jacques Pelletier du Mans, also spelled Peletier ( la, Iacobus Peletarius Cenomani, 25 July 1517 – 17 July 1582) was a humanist, poet and mathematician of the French Renaissance. Born in Le Mans into a bourgeois family, he studied at the Co ...

. Then he studied three years in the Faculty of Law in

Toulouse Toulouse ( , ; oc, Tolosa ) is the prefecture of the French department of Haute-Garonne and of the larger region of Occitania. The city is on the banks of the River Garonne, from the Mediterranean Sea, from the Atlantic Ocean and from Par ...

. In 1686, he went to

Paris Paris () is the capital and most populous city of France, with an estimated population of 2,165,423 residents in 2019 in an area of more than 105 km² (41 sq mi), making it the 30th most densely populated city in the world in 2020. S ...

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 (French: ''Académie des 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 ...

. 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 p ...

Rochefort Rochefort () may refer to: Places France * Rochefort, Charente-Maritime, in the Charente-Maritime department ** Arsenal de Rochefort, a former naval base and dockyard * Rochefort, Savoie in the Savoie department * Rochefort-du-Gard, in the Ga ...

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 Jesuit College and the University of Caen, where he received his M.A. in 1682. He took Holy Orders the following year. Varignon gained his first ex ...

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 Gregory's series, is an infinite Taylor series expansion of the inverse tangent function. It was discovered in 1668 by James Gregory. It was re-rediscovered a few years later by Gottfried Leibniz, who re obtained the Leibniz formula for π as the ...

for arctangent, but only 112 decimals were correct. This remained the record until 1789, when

Jurij Vega Baron Jurij Bartolomej Vega (also Veha; la, Georgius Bartholomaei Vecha; german: Georg Freiherr von Vega; born ''Vehovec'', March 23, 1754 – September 26, 1802) was a Slovene mathematician, physicist and artillery officer. Early life Bor ...

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