In: Kibernetika, USSR, Vol. 17, No. 4, pages 33-41. July-August 1981. In Russian.
Also in: Cybernetics, Vol. 17, No. 4, pages 464-473. July-August 1981. English translation.
Also in: Gruska, J.; Chytil, M.: Lecture Notes in Computer Science, Vol. 118: Mathematical Foundation of Computer Science, Proc. of the 10th Symp., Strbske Pleso 1981, pages 244-251. Berlin, New York: Springer-Verlag, 1981. English translation.