**Charles Pisot** was a French mathematician who worked in number theory.

- This town, where Charles was born, is 25 km south west of Strasbourg and, with the rest of Alsace, had been annexed by Germany in 1871 following the Franco-German war of 1870-71.
- The school that Pisot attended was in Obernai and he was fortunate that his first teacher of mathematics was an enthusiast for the subject.
- The teacher went on to explain more about continued fractions to Pisot who became fascinated - continued fractions would play a large role in his mathematical work from that time on.
- Pisot sat the entrance examinations for the École Normale Supérieure on rue d'Ulm and entered this prestigious university in 1929.
- The director tried to persuade Pisot to undertake research in complex variables but Pisot said he would rather become a school teacher than to be a researcher in complex variables.
- But curiously, at the time when Charles Pisot began his research career the French contributions to the Theory of Numbers were in an unfortunate eclipse.
- However, Pisot did work on number theory for his doctorate, but he had to find his own problems to tackle.
- Pisot had published a number of short papers before submitting his thesis: Sur une propriété caractéristique de certains entiers algébraique Ⓣ(On a characteristic property of some algebraic integers) (1936), Sur certaines propriétés caractéristiques des nombres algébraique Ⓣ(On some characteristic properties of algebraic numbers) (1936), Sur la répartition modulo 1 des puissances successive d'un même nombre Ⓣ(On the distribution of successive powers modulo 1 of the same number) (1937), and Sur la répartition modulo 1 Ⓣ(On the distribution modulo 1) (1937).
- These papers contain his famous 'Pisot numbers', sometimes called the Pisot-Vijayaraghavan numbers', but denoted by SSS by Pisot himself to honour Raphaël Salem.
- After the fall of France, Pisot was offered a position in a French university but instead he chose to move to Germany.
- Although this may sound a little strange, one must remember that Pisot was born in Alsace at a time when it was part of Germany.
- Pisot, now known in Germany as Karl Pisot, was given a temporary appointment at the University of Freiburg in 1940.
- The official died, and in October 1944 Pisot was still not a German citizen.
- Constantin Carathéodory had recommended Pisot for a permanent appointment in 1942 but the citizenship issue prevented this taking place.
- By the summer of 1944 Pisot was working with Wilhelm Süss at his Research Institute at Freiburg.
- In early 1945 Süss was arguing strongly for Pisot's German citizenship to be settled so that he could be appointed to the University of Freiburg, the Research Institute at Oberwolfach, or to both.
- Pisot went with Süss, and other mathematicians from military establishments and colleagues from Freiburg, to Oberwolfach and lived there until the end of the war.
- After the war ended, Pisot returned to France and, in 1946, was appointed as an assistant professor at the University of Bordeaux.
- In addition to working at the Faculty of Science, Pisot also taught at the École Polytechnique.
- In Paris, Pisot worked with his colleagues Hubert Delange and Georges Poitou organising the Delange-Pisot-Poitou seminar.
- In the summer of 1963, Pisot gave a series of lectures at the University of Montreal in Canada.
- Raphaël Salem, who was a major influence on Pisot, died in the summer of 1963 and Pisot dedicated his book to Salem.
- Pisot made the 'Pisot dth root conjecture' which led to considerable progress.
- Of course, French mathematics was for many years dominated by the Bourbaki group of mathematicians and Pisot was invited to join them.
- As we have explained, students were always of major importance to Pisot.
- But this underestimates the influence of Charles Pisot on the Theory of Numbers by only considering those who were directly his pupils.
- Patrice Philippon, who submitted his thesis in 1984, was a pupil of Daniel Bertrand (submitted 1977), who was a pupil of Michel Waldschmidt (submitted 1972), who was a pupil of Jean Fresnel (submitted 1967), who was a pupil of Yvette Amice (submitted 1965), who was herself a pupil of Charles Pisot.
- Pisot retired from his positions in Paris in 1979.

Born 2 March 1910, Obernai, Alsace-Moselle, Germany (now France). Died 7 March 1984, Paris, France.

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