Complex sentence: Leopold Henri Haimson is a historian and professor emeritus of Columbia University, where he worked since 1965 as a professor of Russian history and a member of the Russian Institute.
Split into simpler sentences: Leopold Henri Haimson (?-present) is a historian and professor emeritus of Columbia University. Haimson joined the faculty at Columbia in 1965 as a professor of Russian history and a member of the Russian Institute.