Panchromatic patterns by paths

Abstract

Let $H=(V_H,A_H)$ be a digraph, possibly with loops, and let $D=(V_D, A_D)$ be a loopless multidigraph with a colouring of its arcs $c: A_D \rightarrow V_H$. An $H$-path of $D$ is a path $(v_0, \dots, v_n)$ of $D$ such that $(c(v_{i-1}, v_i), c(v_i,v_{i+1}))$ is an arc of $H$ for every $1 \le i \le n-1$. For $u, v \in V_D$, we say that $u$ reaches $v$ by $H$-paths if there exists an $H$-path from $u$ to $v$ in $D$. A subset $S \subseteq V_D$ is $H$-absorbent of $D$ if every vertex in $V_D-S$ reaches by $H$-paths some vertex in $S$, and it is $H$-independent if no vertex in $S$ can reach another (different) vertex in $S$ by $H$-pahts. An $H$-kernel is an independent by $H$-paths and absorbent by $H$-paths subset of $V_D$. We define $\tilde{\mathscr{B}}_1$ as the set of digraphs $H$ such that any $H$-arc-coloured tournament has an $H$-absorbent by paths vertex; the set $\tilde{\mathscr{B}}_2$ consists of the digraphs $H$ such that any $H$-arc-coloured digraph $D$ has an independent, $H$-absorbent by paths set; analogously, the set $\tilde{\mathscr{B}}_3$ is the set of digraphs $H$ such that every $H$-arc-coloured digraph $D$ contains an $H$-kernel by paths. In this work, we present a characterization of $\tilde{\mathscr{B}}_2$, and provide structural properties of the digraphs in $\tilde{\mathscr{B}}_3$ which settle up its characterization except for the analysis of a single digraph on three vertices.