Recall from Chapter 1 that adigraph is a finite setν of objects calledvertices together with a finite set of directed edges, orarcs, which areordered pairs of vertices. It is like a graph except that each edge is allocated a direction — one Vertex is designated astart and the other isafinish. An arc directed from Starts to finisht is denoted by(s, t), or simplyst. It is important to observe that, unlike a graph, a digraph can have two arcs with the same endpoints, provided they are directed in opposite ways. But we shall not allowmultiple arcs orloops.

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Authors and Affiliations

1. Department of Mathematics, Southern Illinois University, Carbondale, IL, 62901-4408, USA

   W. D. Wallis

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© 2007 Second Edition, Birkhäuser Boston

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