Joseph-Louis Lagrange

See also: Lagrange

Joseph Louis, count de Lagrange (in Italy N Giuseppe Lodovico Lagrangia ), born with Turin the January 25th 1736 and died in Paris the April 10th 1813, is a mathematician. Born in Italy, but from French family by her father, it spent 30 years in Piedmont, then 21 years in Berlin and the remainder of its days in Paris.

Its life

Named very young professor at the artillery school of Turin in 1755, it founds in 1758 there the Académie of Turin which publishes its first work. It is allowed with the Académie of Berlin by Euler, which it succeeds as president. Transferred to Paris, where it had made publish its Analytical mechanics (1787), little before the French revolution, it must with its genius escape measurements from repression against the foreigners. Special decrees of the Comité of public hello enable him to continue to exert its functions. Become associated foreigner of the Academy of Science in 1772, he is director of the Academy in 1788 and member of the section of mathematics in 1795.

Especially known to have introduced the analytical method in geometry, he did not study all the branches of the Mathématiques and left less of them important work as well in Géométrie as in Trigonométrie and Mécanique.

He is buried with the the Pantheon of Paris.

Its work

Founder of the calculation of the variations with Euler and the theory of the quadratic forms, it shows the Théorème of Wilson on the prime numbers and the Conjecture of Bachet about the decomposition of an entirety in four squares. Its name appears everywhere in mathematics. One owes him the theorem of Lagrange on the theory of the groups, another on the continuous fractions, the differential equation of Lagrange.

In physics, by specifying the Principle of less action, with the calculation of the variations about 1756, he invents the function of Lagrange, which checks the equations of Lagrange, then develops the Analytical mechanics , about 1788. He begins also important research on the Problème of the three bodies.

He works out the metric system with Lavoisier during the Révolution. He is founding member of the Bureau of longitudes (1795) with, inter alia, Pierre-Simon Laplace and Jean-Dominique Cassini (Cassini IV). He takes part in the creation of the National university (1795) with Joseph Lakanal, of the Polytechnic school (1794) with Gaspard Monge and Antoine-François Fourcroy, where he teaches as of 1797. He is also the founder of the Academy of Turin (1758).

Principal publications

  • Reflections on the algebraic resolution (1771). This memory inspired Abel and Welsh.
  • Analytical mechanics (1787) in Works of Lagrange , published by Joseph-Alfred Serret, Paris 1867, vol. 11 and 12 (Text on line). Lagrange tries an entirely algebraic approach of mechanics, releasing starting from the principle of virtual work the concepts of Degree of freedom and Intégrabilité. It also outlines the relationship between discrete systems (finished number of masses moving or balance) and continuous systems (fluid).
  • Treated analytical functions . The author defines the functions as convergent developments of Taylor-Lagrange, which enables him to define Dérivée and primitive in an original way, and to raise the problem of the multiple determinations of a function in a point. It is the birth certificate of the theory of the functions of the complex variable, which will be developed on this basis by Cauchy, Poisson and Legendre.

Homages

  • Its name is registered on the Eiffel Tower.
  • the Astéroïde (1006) Lagrangea was named in its honor.

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