Elsevier, Discrete Mathematics, 1-3(187), p. 171-183, 1998
DOI: 10.1016/s0012-365x(97)00233-1
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A graph has a unilateral orientation if its edges can be oriented such that for every two vertices u and v there exists either a path from u to v or a path from v to u. If G is a graph with a unilateral orientation, then the forced unilateral orientation number of G is defined to be the minimum cardinality of a subset of the set of edges for which there is an assignment of directions that has a unique extension to a unilateral orientation of G. This paper gives a general lower bound for the forced unilateral orientation number and shows that the unilateral orientation number of a graph of size m, order n, and having edge connectivity 1 is equal to m − n + 2. A few other related problems are discussed.