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National science information systems : a guide to science information systems in Bulgaria, Czechoslovakia, Hungary, Poland, Romania, and Yugoslavia
Kraus, David H., (1972)
The system of scientific information on social sciences in the People's Republic of Bulgaria
Gabrovska, S., (1977)
Sistema organov informacii v oblasti selʹskogo i lesnogo chozjajstva v Bolgarii i perspektivy ce razvitija
Kolarova, M., (1970)