Question: What profession has both Ivan Fesenko and Mark Krein held?
Sentence (from "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.
Relevant evidence: Yes