Various Theorems on Tournaments

In this thesis we prove a variety of theorems on tournaments. A \emph{prime} tournament is a tournament $G$ such that there is no $X\subseteq V(G)$, $1 < |X| < |V(G)|$,suchthatforeveryvertex$v \in V(G) \minus X$,either$v \ra x$forall$x \in X$or$x \ra v$forall$x \in X$. 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 $5 \le |V(H)| < |V(G)|$thereexistsaprimesubtournamentof$G$with$|V(H)| + 1$ vertices that has a subtournament isomorphic to $H$. We next prove that for any two cyclic triangles $C$, $C^{\mathrm{\prime }}$ in a prime tournament $G$, there is a sequence of cyclic triangles $C_1,\mathrm{.}\mathrm{.}\mathrm{.},C_n$ such that $C_1=C$, $C_n=C^{\mathrm{\prime }}$, 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.