Columbia subject headings:
Bulgaria--Research--Soviet Union--History--Congresses.
Bulgaria--Study and teaching (Higher)--Soviet Union--History--Congresses.
Harvard subject headings:
Bulgaria--Research--Soviet Union--History--Congresses.
Bulgaria--Study and teaching (Higher)--Soviet Union--History--Congresses.
Bulgaria--Study and teaching--Soviet Union.
Princeton subject headings:
Bulgaria--Research--Soviet Union--History--Congresses.
Bulgaria--Study and teaching (Higher)--Soviet Union--History--Congresses.