Columbia subject headings:
Authors, German--20th century--Biography.
Hardt, Ernst, 1876-1947--Correspondence.
Harvard subject headings:
Authors, German--20th century--Biography
Hardt, Ernst, 1876-1947--Correspondence.
Princeton subject headings:
Authors, German--20th century--Correspondence.
Hardt, Ernst, 1876-1947--Correspondence.