A unilateral digraph, also called a unilaterally connected digraph, is a directed graph in which, for any two vertices, at least one of the vertices is reachable
from the other by a directed path (Harary and Palmer 1973, p. 261).
Harary and Palmer (1973, p. 261) give the beginning of the corresponding unlabeled counting series as .
Harary, F. and Palmer, E. M. "A Survey of Graphical Enumeration Problems." In A Survey of Combinatorial Theory (Ed. J. N. Srivastava).
Amsterdam, Netherlands: North-Holland, pp. 259-275, 1973.
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