English

The second out-neighbourhood for local tournaments

Combinatorics 2018-12-06 v1

Abstract

Sullivan stated the conjectures: (1) every oriented graph DD has a vertex xx such that d++(x)d(x)d^{++}(x)\geq d^{-}(x); (2) every oriented graph DD has a vertex xx such that d++(x)+d+(x)2d(x)d^{++}(x)+d^{+}(x)\geq 2d^{-}(x). In this paper, we prove that these conjectures hold for local tournaments. In particular, for a local tournament DD, we prove that DD has at least two vertices satisfying (1)(1) if DD has no vertex of in-degree zero. And, for a local tournament DD, we prove that either there exist two vertices satisfying (2)(2) or there exists a vertex vv satisfying d++(v)+d+(v)2d(v)+2d^{++}(v)+d^{+}(v)\geq 2d^{-}(v)+2 if DD has no vertex of in-degree zero.

Keywords

Cite

@article{arxiv.1812.01800,
  title  = {The second out-neighbourhood for local tournaments},
  author = {Ruijuan Li and Juanjuan Liang},
  journal= {arXiv preprint arXiv:1812.01800},
  year   = {2018}
}