Birkhoff, George David, 1884-1944; Flexner, Abraham, 1866-1959(Shelby White and Leon Levy Archives Center, Institute for Advanced Study, Princeton, NJ, USA, )
Courant, Richard, 1888-1972; Flexner, Abraham, 1866-1959(Shelby White and Leon Levy Archives Center, Institute for Advanced Study, Princeton, NJ, USA, 1932-1935)