Weak and strong local irregularity of digraphs

Abstract

Local Irregularity Conjecture states that every simple connected graph, except special cacti, can be decomposed into at most three locally irregular graphs, i.e., graphs in which adjacent vertices have different degrees. The connected minimization problem, finding the minimum number $k$ such that a graph can be decomposed into $k$ locally irregular graphs, is known to be NP-hard in general (Baudon, Bensmail, and Sopena, 2015). This naturally raises interest in the study of related problems. Among others, the concept of local irregularity was defined for digraphs in several different ways. In this paper we present the following new methods of defining a locally irregular digraph. The first one, weak local irregularity, is based on distinguishing adjacent vertices by indegree-outdegree pairs, and the second one, strong local irregularity, asks for different balanced degrees (i.e., difference between the outdegree and the indegree of a vertex) of adjacent vertices. For both of these irregularities, we define locally irregular decompositions and colorings of digraphs. We discuss relation of these concept to others, which were studied previously, and provide related conjectures on the minimum number of colors in weak and strong locally irregular colorings. We support these conjectures with new results, using the chromatic and structural properties of digraphs and their skeletons (Eulerian and symmetric digraphs, orientations of regular graphs, cacti, etc.).