Various Theorems on Tournaments

Abstract

In this thesis we prove a variety of theorems on tournaments. A \emph{prime} tournament is a tournament $G$ such that there is no $X \subseteq V(G)$, $1 < |X| < |V(G)|$, such that for every vertex $v \in V(G) \minus X$, either $v \ra x$ for all $x \in X$ or $x \ra v$ for all $x \in X$. First, we prove that given a prime tournament $G$ which is not in one of three special families of tournaments, for any prime subtournament $H$ of $G$ with $5 \le |V(H)| < |V(G)|$ there exists a prime subtournament of $G$ with $|V(H)| + 1$ vertices that has a subtournament isomorphic to $H$. We next prove that for any two cyclic triangles $C$, $C^\prime$ in a prime tournament $G$, there is a sequence of cyclic triangles $C_1,...,C_n$ such that $C_1 = C$, $C_n = C^\prime$, and $C_i$ shares an edge with $C_{i+1}$ for all $1 \le i \le n-1$. Next, we consider what we call \emph{matching tournaments}, tournaments whose vertices can be ordered in a horizontal line so that every vertex is the head or tail of at most one edge that points right-to-left. We determine the conditions under which a tournament can have two different orderings satisfying the above conditions. We also prove that there are infinitely many minimal tournaments that are not matching tournaments. Finally, we consider the tournaments $K_n$ and $K_n^\ast$, which are obtained from the transitive tournament with $n$ vertices by reversing the edge from the second vertex to the last vertex and from the first vertex to the second-to-last vertex, respectively. We prove a structure theorem describing tournaments which exclude $K_n$ and $K_n^\ast$ as subtournaments.