Elusive properties of countably infinite graphs

Abstract

A graph property is elusive (or evasive) if any algorithm testing it by asking questions of the form ''Is there an edge between vertices x and y?'' must, in the worst case, examine all pairs of vertices. Elusiveness for infinite vertex sets has been first studied by Csern\'ak and Soukup, who proved that the long-standing Aanderaa-Karp-Rosenberg Conjecture -- which states that every nontrivial monotone graph property is elusive -- fails for infinite vertex sets. We extend their work by giving a closer look to the case when the vertex set is countably infinite and the ''algorithm'' terminates after infinitely many steps. Among others, we prove that connectedness is elusive, which strengthens a result of Csern\'ak and Soukup. We give counterexamples to the infinite version of the Aanderaa-Karp-Rosenberg Conjecture even if the ''algorithm'' is required to terminate after infinitely many steps, which strengthens results of Csern\'ak and Soukup.