Which Russian mathmatician contributed to number theory, Grigori Perelman or Ivan Fesenko?
Ivan Fesenko Ivan Fesenko (Russian: Иван Борисович Фесенко ; born 1962) is a mathematician working in number theory and other areas of modern mathematics.
Nikolay Nekhoroshev Nikolai Nikolaevich Nekhoroshev (Russian: Николай Николаевич Нехорошев ; 2 October 1946 – 18 October 2008) was a prominent Soviet Russian mathematician, specializing in classical mechanics and dynamical systems. His research concerned Hamiltonian mechanics, perturbation theory, celestial mechanics, integrable systems, dynamical systems, the quasiclassical approximation, and singularity theory. He proved, in particular, a stability result in KAM-theory stating that, under certain conditions, solutions of nearly integrable systems stay close to invariant tori for exponentially long times .