Flexibility of planar graphs—Sharpening the tools to get lists of size four

Abstract

A graph where each vertex v has a list L(v) of available colors is L-colorable if there is a proper coloring such that the color of v is in L(v) for each v. A graph is k-choosable if every assignment L of at least k colors to each vertex guarantees an L-coloring. Given a list assignment L, an L-request for a vertex v is a color c∈L(v). In this paper, we look at a variant of the widely studied class of precoloring extension problems from Dvořák, Norin, and Postle (J. Graph Theory, 2019), wherein one must satisfy “enough”, as opposed to all, of the requested set of precolors. A graph G is ɛ-flexible for list sizek if for any k-list assignment L, and any set S of L-requests, there is an L-coloring of G satisfying ɛ-fraction of the requests in S.It is conjectured that planar graphs are ɛ-flexible for list size 5, yet it is proved only for list size 6 and for certain subclasses of planar graphs. We give a stronger version of the main tool used in the proofs of the aforementioned results. By doing so, we improve upon a result by Masařík and show that planar graphs without K4− are ɛ-flexible for list size 5. We also prove that planar graphs without 4-cycles and 3-cycle distance at least 2 are ɛ-flexible for list size 4. Finally, we introduce a new (slightly weaker) form of ɛ-flexibility where each vertex has exactly one request. In that setting, we provide a stronger tool and we demonstrate its usefulness to further extend the class of graphs that are ɛ-flexible for list size 5.