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How old did Fermat become? (English) Zbl 1001.01006
There are numerous questions surrounding the life of Pierre de Fermat. Although we are certain that his birthplace was Beaumont-de-Lomagne, about 55 km north-west of Toulouse, we do not know for sure when he was born. Problems continue with the name and background of his mother, since it is known that his father Dominique, following the death of his first wife, Francoise Cazeneuve, married a second time. Indeed, this second marriage, to the noblewoman Claire de Long, reflected the status Dominique had acquired as a successful businessman, his first wife having been the daughter of a local grocer. Finally, on account of the problem with Pierre de Fermat’s date of birth there are also uncertainties over his age at the time of his death in 1665. Put simply, he might have been 57 or 63 years old.
As the author points out, the first commentator to determine Fermat’s exact date of death was the Italian mathematician and historian of mathematics, Gugliemo Libri, who in the mid-nineteenth century found documents establishing that he had died in Castres on 12 January 1665. This date is also corroborated by the epitaph on the tombstone once contained in the mausoleum of the Fermat family in Toulouse. Already before Libri’s time, a local historian in Beaumont, Louis Taupiac, had discovered evidence that Fermat had been baptised on 20 August 1601. On the question of the identity of Fermat’s mother, Taupiac at first opted for Cazeneuve, but later on the basis of a genealogy he was able to inspect decided on de Long. The biggest problem, however, is the tombstone. It states quite explicitly that Fermat was aged 57 when he died. Clearly, either the age at death or the date of birth must be incorrect.
The author finds strong circumstantial evidence that the source of the discrepancy lies with the hitherto accepted date of birth. Most important here is the tombstone, now housed in the Musée des Augustins in Toulouse. The mathematically well-informed and learned character of the epitaph clearly points to the great mathematician’s son and editor of his works, Samuel de Fermat, as being the author of the inscription. On this assumption it is highly unlikely that the details of Fermat’s life which it contains should be mistaken - otherwise one would have to accept that the stonemason had received false information or that his work had not been checked. But if the dates of the epitaph are correct, then Fermat must have been born in 1607/8, and not in 1601.
Further clues also lend weight to the author’s interpretation. On the basis of the earlier date there would be inexplicable gaps in Fermat’s vita. He would have been unusually late not only in beginning his mathematical career but also in marrying. This leads the author to speculate that Fermat’s mother was Claire de Long and that the records of the baptism in 1601 refer to an eponymous son of Dominique Fermat’s first wife, who died prematurely. As the author explains, not only was there a high rate of infant mortality, but also a woman of Cazeneuve’s background would have been unlikely to have lived beyond her mid-thirties. In addition, it was not unusual for children of a later marriage to be named after non-surviving offspring of an earlier one.
In fact, the author finds that the strongest evidence points to Pierre de Fermat’s year of birth being 1607. On this assumption we find that he began his creative mathematics at the age of 21 in 1628 and married Louise de Long at the age of 24 in 1631. These ages, and not those previously assumed, correspond to the sociological and scientific norms of the time.

MSC:
01A45 History of mathematics in the 17th century
01A70 Biographies, obituaries, personalia, bibliographies
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