**Thomas Clausen** was a Danish mathematician who wrote over 150 papers on pure mathematics, applied mathematics, astronomy and geophysics.

- When he was twelve years old, Thomas began working for the priest in the neighbouring parish, Georg Holst.
- Thomas was employed to look after cattle, but Holst quickly realised that he was very intelligent so while still working with the cattle, Thomas also attended the local school.
- Despite being unable to read or write when he started his lessons, Thomas quickly progressed showing a remarkable aptitude for mathematics.
- Holst was an amateur astronomer and mathematician and was able to teach Clausen these subjects as well as Latin and Greek.
- Clausen also studied several languages on his own, in particular French, English and Italian.
- In around 1819 Clausen began working for Schumacher at the observatory in Altona.
- Peter Andreas Hansen began working for Schumacher in 1821 and he played a role in increasing Clausen's mathematical knowledge.
- Clausen published his first paper Berechnung der Sternbedeckungen vom Monde zur Bestimmung der geographischen Länge Ⓣ(Calculating occultations by the moon to determine longitude) in 1824.
- Clausen became an assistant at Altona Observatory in 1824 and in October of that year he met Carl Friedrich Gauss, who was conducting geodesic measurements nearby, for the first time.
- Things were not going well between Schumacher and Clausen, however, and when Clausen broke an expensive barometer this was the last straw - Schumacher told Clausen to leave just before Christmas 1824.
- Now Clausen was in considerable difficulty but, remembering how well he had got on with Gauss, he travelled to Göttingen to see him.
- The reluctance seems to have been on account of Clausen lacking breeding rather than on account of his work.
- After his first paper, mentioned above, Clausen published two further papers in Volume 2 of Astronomische Nachrichten in 1824: Mondssterne in Altona beobachtet Ⓣ(Observations of the moon and stars made in Altona), and Anzeige von Druckfehlern Ⓣ(Display of misprints).
- Three further papers appeared in 1825: Längendifferenzen aus Mondsculminationen Ⓣ(Calculating distances from lunar observations); Auszug aus einem Briefe des Herrn Thomas Clausen an den Herausgeber Ⓣ(Extract from a letter of Thomas Clausen to the editor); and Resultate der Mondssterne beobachtungen Ⓣ(Results observations of the moon and stars).
- There was no way that Schumacher would allow his niece to marry someone of Clausen's lowly class so he tried to get him to leave Altona by arranging another position for him.
- Schumacher suggested Clausen for the position, and the offer was accepted.
- Clausen was delighted when informed of the job offer and wrote a letter of acceptance on 3 November.
- Although von Steinheil was weaker than Clausen as a mathematician, he had the advantage of being rich and astronomers always need lots of money to carry out their research.
- Clausen's appointment was delayed, putting him in an extremely difficult position since he had resigned from Altona although he was given some money to compensate for the fact that he had a signed contract.
- The difficulty over his appointment was solved at last with both Clausen and von Steinheil being employed at the Munich Optical Institute.
- Clausen started work in December 1828.
- Towards the end of 1832 Clausen suffered a severe disappointment when he learnt that he would never succeed Fraunhofer.
- Clausen's work was recognised by many of the top scientists of the day including Olbers, Gauss, Bessel, Hansen, Crelle, von Humboldt and Arago.
- In 1840 he published a theorem on the Bernoulli numbers which had been proved around the same time by Karl von Staudt - today it is called the von Staudt-Clausen theorem.
- On 10 August 1842, Schumacher wrote to Gauss saying that Clausen had proved the nonexistence of orthogonal Latin squares of order 6 by dividing such Latin squares into 17 families.
- Sadly details of Clausen's proof have never been found although the remarkable combinatorial abilities he displayed leads one to believe that he did have a proof.
- The statement of this theorem is an afterthought to a paper in which Jacobi responds to the published correction by Thomas Clausen (1842) of an earlier paper by Jacobi (1836).
- As soon as Clausen had arrived back in Altona at the beginning of 1840, Schumacher had tried to arrange a position for him at the Observatory in Tartu, at that time part of the Russian Empire and known as Derpt (similar to the earlier German name of Dorpat).
- Johann Heinrich Mädler, the director of the Observatory, would have preferred to employ Johann Gottfried Galle who, unlike Clausen, had an exceptional reputation as an observer.
- Galle did not want to go to Tartu and there followed a lengthy correspondence between Mädler, Schumacher and Gauss as to Clausen's suitability.
- Schumacher and Bessel worried about how Clausen and Mädler would get along.
- Two years later Clausen received an honorary doctorate from the University of Königsberg.
- He had been put forward for this honour by Bessel, who thought very highly of Clausen's extraordinary talents.
- If Clausen had struggled to get positions earlier, this was no longer the case for he received an offer to go to the Observatory in Pulkowa, the leading Russian observatory.
- Clausen wrote over 150 papers on pure mathematics, applied mathematics, astronomy and geophysics.
- Clausen also gave a new method of factorising numbers.
- Clausen's first love was mathematics, particularly number theory, although he had never received a formal mathematical training.

Born 16 January 1801, Snogbaek, Denmark. Died 23 May 1885, Dorpat, Russia (now Tartu, Estonia).

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Astronomy, Origin Denmark, Special Numbers And Numerals

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