Carl Ludwig Siegel

German mathematician specialising in number theory and celestial mechanics, known for, amongst other things, his contributions to the Thue–Siegel–Roth theorem in Diophantine approximation and the Siegel mass formula for quadratic forms. He was named as one of the most important mathematicians of the 20th century.

Amongst his teachers were Max Planck and Ferdinand Georg Frobenius, whose influence made the young Siegel abandon astronomy and turn towards number theory instead. His best student was Jürgen Moser, one of the founders of KAM theory (Kolmogorov–Arnold–Moser), which lies at the foundations of chaos theory. Another notable student was Kurt Mahler, the number theorist.

Siegel was an antimilitarist, and in 1917, during World War I he was committed to a psychiatric institute as a conscientious objector. According to his own words, he withstood the experience only because of his support from Edmund Landau, whose father had a clinic in the neighborhood. After the end of World War I, he enrolled at the Georg-August University of Göttingen, studying under Landau, who was his doctoral thesis supervisor (Ph.D. in 1920). He stayed in Göttingen as a teaching and research assistant; many of his groundbreaking results were published during this period.

He died on 4 April 1981, aged 84, in Göttingen, West Germany.