Avoidability of Digraphs with Height Functions and Orientations of $C_4$

Abstract

A digraph $F$ is avoidable if, for every positive integer $k$, there exists an integer $d$ such that every digraph of minimum out-degree at least $d$ contains an $F$-free subdigraph of minimum out-degree at least $k$. We prove that no digraph admitting a height function is avoidable, where a height function increases by one along every arc. This answers a question of Christoph, Janzer, Petrova, and Steiner and, together with an additional avoidance argument, determines which orientations of $C_4$ are avoidable. Motivated by a further question of Christoph, Janzer, Petrova, and Steiner, we also study Eulerian avoidability, in which the host digraph is required to have equal in-degree and out-degree at every vertex. We show that no one-directed complete bipartite digraph with nonempty parts is Eulerian-avoidable. In contrast, the orientation of $C_4$ consisting of two directed paths of length two with common endpoints is Eulerian-avoidable. Consequently, the one-directed complete bipartite orientation is the only orientation of $C_4$ that is not Eulerian-avoidable, completing the classification of $C_4$-orientations.