English

Tournaments with maximal decomposability

Combinatorics 2021-02-05 v1

Abstract

Given a tournament TT, a module of TT is a subset MM of V(T)V(T) such that for x,yMx, y\in M and vV(T)Mv\in V(T)\setminus M, (x,v)A(T)(x,v)\in A(T) if and only if (y,v)A(T)(y,v)\in A(T). The trivial modules of TT are \emptyset, {u}\{u\} (uV(T))(u\in V(T)) and V(T)V(T). The tournament TT is indecomposable if all its modules are trivial; otherwise it is decomposable. The decomposability index of TT, denoted by δ(T)\delta(T), is the smallest number of arcs of TT that must be reversed to make TT indecomposable. In a previous paper, we proved that for n5n \geq 5, we have δ(n)=n+14\delta(n) = \left\lceil \frac{n+1}{4} \right\rceil, where δ(n)\delta(n) is the maximum of δ(T)\delta(T) over the tournaments TT with nn vertices. In this paper, we characterize the tournaments TT with δ\delta-maximal decomposability, i.e., such that δ(T)=δ(T)\delta(T)=\delta(\vert T\vert).

Keywords

Cite

@article{arxiv.2102.02350,
  title  = {Tournaments with maximal decomposability},
  author = {Cherifa Ben Salha},
  journal= {arXiv preprint arXiv:2102.02350},
  year   = {2021}
}

Comments

14 pages with 2 figures. arXiv admin note: text overlap with arXiv:2003.06503

R2 v1 2026-06-23T22:49:09.466Z