This is a bit delayed, but WaPo only had the obit today. Anyway, on June 3, Vladimir Igorevich Arnol'd died of peritonitis in Paris at age 72. He was arguably the greatest living nonlinear dynamics mathematician and also the greatest still living to have come out of the old Soviet Union. A protegee of the late Andrei Kolmogorov, he finally solved the 13th of the Hilbert unresolved math problems from 1900. It involved the stability of nonlinear dynamical systems, including planetary motion. He wrote on catastrophe theory, never buying into either the excessive hype about it during its fad, while then avoiding the excessive dissing of it that came later.
He was the recipient of the Crafoord Prize, the Shaw Prize, and the Wolf Prize. He would have received the most prestigious of all math prizes, the Fields Medal, in 1974, but was forbidden by the Soviet government. He had signed letters protesting the suppression of political dissidents, although he was not one beyond signing these letters. He was a man of principle as well as of profound intellect.