to the United States, where he joined the Institute for Advanced Study in Princeton, where he had already spent a sabbatical in 1935. He returned to Göttingen only after World War II, when he accepted a post as professor in 1951, which he kept until his retirement in 1959. In 1968 he was elected a foreign associate of the U.S. National Academy of Sciences. Career Siegel's work on number theory, diophantine equations, and celestial mechanics in particular won him numerous honours. In 1978, he was awarded the first Wolf Prize in Mathematics, one of the most prestigious in the field. When the prize committee decided to select the greatest living mathematician, the discussion centered around Siegel and Israel Gelfand as the leading candidates. The prize was ultimately split between them. Siegel's work spans analytic number theory; and his theorem on the finiteness of the integer points of curves, for genus > 1, is historically important as a major general result on diophantine equations, when the field was essentially undeveloped. He worked on L-functions, discovering the (presumed illusory) Siegel zero phenomenon. His work, derived from the Hardy–Littlewood circle method on quadratic forms, appeared in the later, adele group theories encompassing the use of theta-functions. The Siegel modular varieties, which describe Siegel modular forms, are recognised as part of the moduli theory of abelian varieties. In all this work the structural implications of analytic methods show through. In the early 1970s Weil gave a series of seminars on the history of number theory prior to the 20th century and he remarked that Siegel once told him that when the first person discovered the simplest case of Faulhaber's formula then, in Siegel's words, "Es gefiel dem lieben Gott." (It pleased the dear Lord.) Siegel was a profound student of the history of mathematics and put his studies to good use in such works as the Riemann–Siegel formula. Works by Siegel: Transcendental numbers, 1949 Analytic functions of several complex variables, Stevens 1949; 2008 pbk editionGesammelte Werke, 3 Bände, Springer 1966 with Jürgen Moser Lectures on Celestial mechanics 1971, based upon the older work Vorlesungen über Himmelsmechanik, Springer 1956On the history of the Frankfurt Mathematics Seminar, Mathematical Intelligencer Vol.1, 1978/9, No. 4Über einige Anwendungen diophantischer Approximationen, Sitzungsberichte der Preussischen Akademie der Wissenschaften 1929 (sein Satz über Endlichkeit Lösungen ganzzahliger Gleichungen)Transzendente Zahlen, BI Hochschultaschenbuch 1967Vorlesungen über Funktionentheorie, 3 Bde. (auch in Bd.3 zu seinen Modulfunktionen, English