Mark Kac (Marek Kac) (1914 - 1984) (pronounced kahts) was a Polish mathematician. He was born in Krzemieniec and died in California, USA.
His main interest was probability. His question "can you hear the shape of a drum?" set off research into spectral theory, with 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.)
His book of popularisation with Stanislaw Ulam is a minor classic.
