What profession has both Ivan Fesenko and Mark Krein held?
Mark Krein: Mark Grigorievich Krein (Ukrainian: Марко Григорович Крейн , Russian: Марк Григо́рьевич Крейн ; 3 April 1907 – 17 October 1989) was a Soviet Jewish mathematician, one of the major figures of the Soviet school of functional analysis. Ivan Fesenko: Ivan Fesenko (Russian: Иван Борисович Фесенко ; born 1962) is a mathematician working in number theory and other areas of modern mathematics.
mathematician