**Sylvestre Lacroix** was a French mathematician who wrote important textbooks on geometry and analysis.

- At this time Monge was officially employed at the École at Mézières but he had been elected to the Académie des Sciences in Paris in 1780 and began to spend most of his time there teaching courses such as the one that Lacroix attended.
- Lacroix was fortunate to be a student of Monge who soon realised that his young pupil had an outstanding talent for mathematics and he used his influence to help him develop his career.
- Monge had replaced Bézout as examiner of naval cadets and this post put him in a position where he could recommend that Lacroix be appointed as Professor of Mathematics at the École Gardes de Marine at Rochefort in 1782.
- Not only did Monge use his influence to obtain this position for Lacroix, but he also acted as his mathematical advisor, recommending that he undertake research on partial differential equations and the calculus of variations.
- Lacroix followed his advice and in 1785, when still only twenty years old, he sent a memoir to Monge who presented it to the Académie des Sciences.
- Lacroix had maintained his early interest in calculating astronomical data, and in the same year he sent his new solar tables to the Academy.
- From Rochefort, Lacroix returned to Paris.
- It was this position which allowed Lacroix to return to teach in Paris but the pure mathematics course at this newly opened school, which attracted most of its students from the nobility, was poorly attended and soon discontinued.
- Lacroix continued to teach astronomy and the theory of probability at the Lycée.
- The Academy of Sciences proposed the topic of its Grand Prix for 1787 to be on the theory of marine insurance, and Lacroix submitted an entry.
- Lacroix moved again to the provinces.
- This time Lacroix went to Besançon where he was appointed as Professor of Mathematics, Physics and Chemistry at the École d'Artillerie.
- Lacroix was appointed to succeed him as examiner at the Artillery Corps on 1 October 1793.
- Lacroix and Hachette assisted Monge in the work for his descriptive geometry course.
- Lacroix was particularly well suited to assist with this course since he had been assembling material on descriptive geometry for several years, and he went on to publish Eléments de géométrie déscriptive.
- On 20 May 1799 Lacroix was elected a member of the Institut, becoming one of the six in the mathematics section on the death of Borda.
- Lacroix was the writer of important textbooks in mathematics and through these he made a major contribution to the teaching of mathematics throughout France and also in other countries.
- The efforts of Lacroix and Legendre to expose or recast the theory of parallels were notable, as were their additions to the theory.

Born 28 April 1765, Paris, France. Died 24 May 1843, Paris, France.

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Analysis, Astronomy

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