**Louis Poinsot** was the inventor of geometrical mechanics, investigating how a system of forces acting on a rigid body could be resolved into a single force and a couple.

- Despite this poor performance in one part of the entrance examinations, Poinsot was accepted for admission to the École Polytechnique.
- From 1794 until 1797 Poinsot was a student at the École Polytechnique but then he left and went to study at the École des Ponts et Chaussée with the intention of becoming an engineer.
- Poinsot seems to have somewhat lacked vision at this stage in his career as to what he wanted for the future.
- Having made rather a mess of his education, Poinsot decided to give up the idea of becoming an engineer and left the École des Ponts et Chaussée to become a mathematics teacher.
- From 1804 until 1809 Poinsot was a mathematics teacher at the Lycée Bonaparte in Paris.
- Poinsot took on another appointment, in addition to the one with the Imperial University, when he accepted the position of assistant professor of analysis and mechanics at the École Polytechnique on 1 November 1809.
- We should say a little about how someone like Poinsot, who had not really made a great success of his education, had moved from school teaching into higher education in the way that he did.
- The reason is that Poinsot had been very active in mathematics even while he was supposed to be studying for an engineering qualification.
- Certainly Poinsot had been very busy during his years as a school teacher of mathematics and by 1809 he had made for himself a fine reputation.
- In 1812 Poinsot arranged for Reynaud to substitute for him in lecturing at the École Polytechnique and, from this time on, he did no further teaching there asking Cauchy to substitute for him after Reynaud.
- In September 1816 the École Polytechnique was reorganised and Poinsot lost the post which he had held in name only for the previous four years.
- By 1816 Poinsot had been elected to the Académie des Sciences for, on 31 May 1813, he had replaced Lagrange in the mathematics section of the Académie.
- The 1816 reorganisation of the École Polytechnique saw Poinsot become admissions examiner and he held this post for ten years.
- He wrote an important work on polyhedra in 1809 (already mentioned above), discovering four new regular polyhedra, two of which appear in Kepler's work of 1619 but Poinsot was unaware of this.
- A mistake was discovered in Poinsot's (and hence Cauchy's) definition in 1990 when an internal inconsistency became apparent.
- In addition Poinsot worked on number theory and on this topic he studied Diophantine equations, how to express numbers as the difference of two squares and primitive roots.
- As well as his research in geometry, Poinsot contributed to its increasing importance by creating a chair of advanced geometry at the Sorbonne in 1846.
- Poinsot intended the chair for Chasles and indeed he was appointed to the new chair which he occupied until his death in 1880.

Born 3 January 1777, Paris, France. Died 5 December 1859, Paris, France.

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Algebra

**O’Connor, John J; Robertson, Edmund F**: MacTutor History of Mathematics Archive