neal young / Hakimi97Orienting

Information Processing Letters 63(5):229235(1997)The paper focuses on two problems: (i) how to orient the edges of an undirected graph in order to maximize the number of ordered vertex pairs $(x,y)$ such that there is a directed path from $x$ to $y$, and (ii) how to orient the edges so as to minimize the number of such pairs. The paper describes a quadratictime algorithm for the first problem, and a proof that the second problem is NPhard to approximate within some constant $1+ε > 1$. The latter proof also shows that the second problem is equivalent to ``comparability graph completion''; neither problem was previously known to be NPhard.
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