Juergen Moser, 71, an American mathematician recognized worldwide for proving a theory on the workings of the solar system, died of cancer Dec. 17 at his home in the Zurich suburb of Schwerzenbach.

"He was one of the leading mathematicians of the postwar era," Konrad Osterwalder, a colleague at Switzerland's prestigious Federal Institute of Technology, wrote in an obituary in the daily Neue Zuercher Zeitung.

In 1994, Dr. Moser was awarded the Wolf Prize in Mathematics for his work in the 1960s analyzing the stability of the solar system.

He is credited with proving a general theory on celestial mechanics by the Russian mathematician Andrei N. Kolmogorov that the gravitational pull of distant planets may have no apparent effect on elliptical orbits in space. Kolmogorov offered no proof, but his student, V.I. Arnold, provided one and Moser another. As a result, the theory is known as Kolmogorov-Arnold-Moser.

Dr. Moser's work was important to the understanding of the solar system and to the development of particle accelerators, which have allowed scientists to test many theories about nature's tiniest building blocks.

Born July 4, 1928, in Koenigsberg, then an eastern outpost of Germany and now the Russian city of Kaliningrad, Dr. Moser studied at the University of Goettingen in West Germany before going to the United States on a Fulbright fellowship in 1953.

Dr. Moser, who became a U.S. citizen, was a research associate at New York University in 1957 when he left to become visiting professor at the Massachusetts Institute of Technology. He returned to NYU in 1960, becoming a professor at the Courant Institute of Mathematical Sciences. He was the institute's director from 1967 to 1970 and remained a professor there until 1980.

Dr. Moser then went back to Europe, becoming a teacher and researcher at the Swiss technology institute. He served as director of its Research Institute for Mathematics until 1995, when he was made professor emeritus.

From 1983 to 1986, he was president of the International Mathematical Union.

Survivors include his wife, Gertrude, two daughters, a stepson and a brother.