On the Distinguishing Number of Cyclic Tournaments: Towards the Albertson-Collins Conjecture

Abstract

A distinguishing $r$-labeling of a digraph $G$ is a mapping $\lambda$ from the set of verticesof $G$ to the set of labels $\{1,\dots ,r\}$ such that no nontrivial automorphism of $G$ preserves all the labels.The distinguishing number $D(G)$ of $G$ is then the smallest $r$ for which $G$ admits a distinguishing $r$-labeling.From a result of Gluck (David Gluck, Trivial set-stabilizers in finite permutation groups,{\em Can. J. Math.} 35(1) (1983), 59--67),it follows that $D(T)=2$ for every cyclic tournament~$T$ of (odd) order $2q+1\ge 3$.Let $V(T)=\{0,\dots ,2q\}$ for every such tournament.Albertson and Collins conjectured in 1999that the canonical 2-labeling ${\lambda }^{\ast }$ given by${\lambda }^{\ast }(i)=1$ if and only if $i\le q$ is distinguishing.We prove that whenever one of the subtournaments of $T$ induced by vertices $\{0,\dots ,q\}$or $\{q+1,\dots ,2q\}$ is rigid, $T$ satisfies Albertson-Collins Conjecture.Using this property, we prove that several classes of cyclic tournaments satisfy Albertson-Collins Conjecture.Moreover, we also prove that every Paley tournament satisfies Albertson-Collins Conjecture.