[2] HARARY F., READ R. C:
Is the null-gгaph a pointless concept?. In: Gгaphs and Combinatorics, Proc. Capital Conf. on Gгaph Theoгy and Combinatoгics, George Washington University, June 1973. Springer Verlag, Beгlin-Heidelberg-New York 1974.
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