English

The indecomposable tournaments $T$ with $\mid W_{5}(T) \mid = \mid T \mid -2$

Combinatorics 2013-07-19 v1

Abstract

We consider a tournament T=(V,A)T=(V, A). For XVX\subseteq V, the subtournament of TT induced by XX is T[X]=(X,A(X×X))T[X] = (X, A \cap (X \times X)). An interval of TT is a subset XX of VV such that for a,bXa, b\in X and xVX x\in V\setminus X, (a,x)A(a,x)\in A if and only if (b,x)A(b,x)\in A. The trivial intervals of TT are \emptyset, {x}(xV)\{x\}(x\in V) and VV. A tournament is indecomposable if all its intervals are trivial. For n2n\geq 2, W2n+1W_{2n+1} denotes the unique indecomposable tournament defined on {0,,2n}\{0,\dots,2n\} such that W2n+1[{0,,2n1}]W_{2n+1}[\{0,\dots,2n-1\}] is the usual total order. Given an indecomposable tournament TT, W5(T)W_{5}(T) denotes the set of vVv\in V such that there is WVW\subseteq V satisfying vWv\in W and T[W]T[W] is isomorphic to W5W_{5}. Latka \cite{BJL} characterized the indecomposable tournaments TT such that W5(T)=W_{5}(T)=\emptyset. The authors \cite{HIK} proved that if W5(T)W_{5}(T)\neq \emptyset, then W5(T)V2\mid W_{5}(T) \mid \geq \mid V \mid -2. In this article, we characterize the indecomposable tournaments TT such that W5(T)=V2\mid W_{5}(T) \mid = \mid V \mid -2.

Keywords

Cite

@article{arxiv.1307.5027,
  title  = {The indecomposable tournaments $T$ with $\mid W_{5}(T) \mid = \mid T \mid -2$},
  author = {Houmem Belkhechine and Imed Boudabbous and Kaouthar Hzami},
  journal= {arXiv preprint arXiv:1307.5027},
  year   = {2013}
}