Columbia subject headings:
Hausdorff, Felix, 1868-1942.
Topological spaces--Congresses.
Topology--Congresses.
Harvard subject headings:
Hausdorff, Felix, 1868-1942.
Topological spaces--Congresses.
Topology--Congresses
Princeton subject headings:
Hausdorff, Felix, 1868-1942--Congresses.
Topology--Congresses.