On the Chromatic Polynomial and Counting DP-Colorings
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The chromatic polynomial of a graph G, denoted P(G,m), is equal to the number of proper m-colorings of G. The list color function of graph G, denoted P_ℓ(G,m), is a list analogue of the chromatic polynomial that has been studied since 1992, primarily through comparisons with the corresponding chromatic polynomial. DP-coloring (also called correspondence coloring) is a generalization of list coloring recently introduced by Dvořák and Postle. In this paper, we introduce a DP-coloring analogue of the chromatic polynomial called the DP color function denoted P_DP(G,m). Motivated by known results related to the list color function, we show that while the DP color function behaves similar to the list color function for some graphs, there are also some surprising differences. For example, Wang, Qian, and Yan recently showed that if G is a connected graph with l edges, then P_ℓ(G,m)=P(G,m) whenever m > l-1/(1+ √(2)), but we will show that for any g ≥ 3 there exists a graph, G, with girth g such that P_DP(G,m) < P(G,m) when m is sufficiently large. We also study the asymptotics of P(G,m) - P_DP(G,m) for a fixed graph G, and we develop techniques to compute P_DP(G,m) exactly for certain classes of graphs such as chordal graphs, unicyclic graphs, and cycles with a chord.
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