Mark Kac ( ; Polish: Marek Kac; August 3, 1914 – October 26, 1984) was a Polish American mathematician. His main interest was probability theory. His question, "Can one hear the shape of a drum?" set off research into spectral theory, the idea of understanding the extent to which the spectrum allows one to read back the geometry. (In the end, the answer was "no", in general.) Biography He was born to a Polish-Jewish family; their town, Kremenets (Polish: "Krzemieniec"), changed hands from the Russian Empire (by then Soviet Ukraine) to Poland after the Peace of Riga, when Kac was a child. Kac completed his Ph.D. in mathematics at the Polish University of Lwów in 1937 under the direction of Hugo Steinhaus. While there, he was a member of the Lwów School of Mathematics. After receiving his degree, he began to look for a position abroad, and in 1938 was granted a scholarship from the Parnas Foundation, which enabled him to go work in the United States. He arrived in New York City in November 1938. With the onset of World War II in Europe, Kac was able to stay in America, while his parents and brother, who had remained in Kremenets, were murdered by the Germans in mass executions in August 1942. From 1939 to 1961, Kac taught at Cornell University, first as an instructor, then from 1943 as an assistant professor and from 1947 as a full professor. While there, he became a naturalized US citizen in 1943. During the 1951–1952 academic year, Kac was on sabbatical at the Institute for Advanced Study. In 1952, Kac, with Theodore H. Berlin, introduced the spherical model of a ferromagnet (a variant of the Ising model) and, with J. C. Ward, found an exact solution of the Ising model using a combinatorial method. In 1961, Kac left Cornell and went to The Rockefeller University in New York City. In the early 1960s, he worked with George Uhlenbeck and P. C. Hemmer on the mathematics of a van der Waals gas. After twenty years at Rockefeller, he moved to the University of Southern California where he spent the rest of his career. Work In his 1966 article titled "Can one hear the shape of the drum", Kac asked the question whether two resonators ("drums") of different geometrical shapes can have exactly the same set of frequencies ("sound tones"). The answer was negative,