Geodesics in Oriented Graphs


A geodesic from a to b in a directed graph is the shortest directed path from a to b. R. C. Entringer made the beautiful conjecture that every graph has an orientation in which every geodesic is unique. Unfortunately this is not true; the aim of this note is to present two different ways of disproving it. First, we show that for a large enough q the Paley graph Qq, has no orientation in which every geodesic is unique, and then we apply a probabilistic argument to show that almost no graph has an orientation with unique geodesies. I am grateful to Adrian Bondy for bringing Entringer's conjecture to my attention. Let q be a prime power and write Fq, for the field of order q. If q is a power of a prime congruent to 1 modulo 4, then the Paley graph Qq, is defined on Fq, by joining a to b if a -b is a quadratic residue. It is well known that Qq is a strongly regular graph of degree (q - 1)/2, in which two adjacent vertices have (q - 1)/4 common neighbours. For the sake of completeness we give detailed proofs of the following folklore lemmas concerning Paley graphs. I am grateful to J. W. S. Cassels for the slick proof of the first. © 1984.

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North-Holland Mathematics Studies