Columbia subject headings:
Hilbert, David, 1862-1943--Correspondence.
Klein, Felix, 1849-1925--Correspondence.
Mathematicians--Correspondence.
Harvard subject headings:
Hilbert, David, 1862-1943--Correspondence.
Klein, Felix, 1849-1925--Correspondence.
Mathematicians--Correspondence.
Princeton subject headings:
Hilbert, David, 1862-1943.
Klein, Felix, 1849-1925.
Mathematicians--Correspondence.