English

$H$-kernels in $H$-colored digraphs without $(\xi_{1}, \xi, \xi_{2})$-$H$-subdivisions of $\overrightarrow{C_{3}}$

Combinatorics 2020-06-09 v1

Abstract

Let HH be a digraph possibly with loops and DD a digraph without loops with a coloring of its arcs c:A(D)V(H)c:A(D) \rightarrow V(H) (DD is said to be an HH-colored digraph). A directed path WW in DD is said to be an HH-path if and only if the consecutive colors encountered on WW form a directed walk in HH. A subset NN of vertices of DD is said to be an HH-kernel if (1) for every pair of different vertices in NN there is no HH-path between them and (2) for every vertex uu in V(DD)\setminusNN there exists an HH-path in DD from uu to NN. Under this definition an HH-kernel is a kernel whenever A(H)=A(H)=\emptyset. The color-class digraph CC\mathscr{C}_C(DD) of DD is the digraph whose vertices are the colors represented in the arcs of DD and (ii,jj) \in AA(CC\mathscr{C}_C(DD)) if and only if there exist two arcs, namely (uu,vv) and (vv,ww) in DD, such that (uu,vv) has color ii and (vv,ww) has color jj. Since not every HH-colored digraph has an HH-kernel and V(CC(D))=V(H)V(\mathscr{C}_C(D))= V(H), the natural question is: what structural properties of CC(D)\mathscr{C}_C(D), with respect to the HH-coloring, imply that DD has an HH-kernel? In this paper we investigate the problem of the existence of an HH-kernel by means of a partition ξ\xi of V(H)V(H) and a partition \{ξ1\xi_1, ξ2\xi_2\} of ξ\xi. We establish conditions on the directed cycles and the directed paths of the digraph DD, with respect to the partition \{ξ1\xi_1, ξ2\xi_2\}. In particular we pay attention to some subestructures produced by the partitions ξ\xi and \{ξ1\xi_1, ξ2\xi_2\}, namely (ξ1,ξ,ξ2)(\xi_{1}, \xi, \xi_{2})-HH-subdivisions of C3\overrightarrow{C_{3}} and (ξ1,ξ,ξ2)(\xi_{1}, \xi, \xi_{2})-HH-subdivisions of P3\overrightarrow{P_{3}}. We give some examples which show that each hypothesis in the main result is tight.

Keywords

Cite

@article{arxiv.2006.03691,
  title  = {$H$-kernels in $H$-colored digraphs without $(\xi_{1}, \xi, \xi_{2})$-$H$-subdivisions of $\overrightarrow{C_{3}}$},
  author = {Felipe Hernández-Lorenzana and Rocío Sánchez-López},
  journal= {arXiv preprint arXiv:2006.03691},
  year   = {2020}
}

Comments

15 pages, 5 figures