Columbia subject headings:
Cartan, Elie, 1869-1951.
Einstein, Albert, 1879-1955.
Physicists--Correspondence.
Unified field theories.
Harvard subject headings:
Cartan, Elie, 1869-1951.
Einstein, Albert, 1879-1955
Physicists--Correspondence
Unified field theories.
Princeton subject headings:
Cartan, Elie, 1869-1951--Biography.
Einstein, Albert, 1879-1955--Biography.
Physicists--Correspondence.
Unified field theories.