Two characterizations of simple circulant tournaments

Abstract

The \textit{acyclic disconnection} $\overrightarrow{\omega }(D)$ (resp. the \textit{directed triangle free disconnection } $\overrightarrow{\omega }_{3}(D)$) of a digraph $D$ is defined as the maximum possible number of connected components of the underlying graph of $D\setminus A(D^{\ast })$ where $D^{\ast }$ is an acyclic (resp. a directed triangle free) subdigraph of $D$. In this paper, we generalize some previous results and solve some problems posed by V. Neumann-Lara (The acyclic disconnection of a digraph, Discrete Math. 197/198 (1999), 617-632). Let $\overrightarrow{C}_{2n+1}(J)$ be a circulant tournament. We prove that $\overrightarrow{C}_{2n+1}(J)$ is $\overrightarrow{% \omega }$-keen and $\overrightarrow{\omega _{3}}$-keen, respectively, and $% \overrightarrow{\omega }(\overrightarrow{C}_{2n+1}(J))=\overrightarrow{% \omega }_{3}(\overrightarrow{C}_{2n+1}(J))=2$ for every $\overrightarrow{C}% _{2n+1}(J)$. Finally, it is showed that $\overrightarrow{\omega }_{3}(% \overrightarrow{C}_{2n+1}(J))=2$, $\overrightarrow{C}_{2n+1}(J)$ is simple and $J$ is aperiodic are equivalent propositions.