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Enumerative Chromatic Choosability


Enumerative chromatic choosability is the property of a graph G that its list color function equals its chromatic polynomial for every positive integer m:

 P_l(G,m)=P(G,m).

Since both sides vanish below the chromatic number chi(G), it is equivalent to require the equality only for m>=chi(G). It implies chromatic choosability, the equality of choice number and chromatic number, but the converse fails (Allred and Mudrock 2026).

A connected graph with chromatic number 2 is enumeratively chromatic-choosable iff its core, obtained by successively deleting vertices of vertex degree 1, is K_1, an even cycle graph, or the complete bipartite graph K_(2,3) (Allred and Mudrock 2026, Theorem 10). For disconnected graphs, the property holds if it holds for every connected component, since both counting functions multiply over components. The converse fails. For example, the graph disjoint union Theta(2,2,4) union K_3 is enumeratively chromatic-choosable although its theta graph component is not. For m<=2, both counting functions of the union vanish because of K_3, while for m>=3 the two functions agree on both components (Allred and Mudrock 2026, Theorem 11).

The theta graph Theta(2,2,2k), consisting of three internally disjoint paths of the indicated lengths joining the same two vertices, is chromatic-choosable but fails enumerative chromatic choosability for k>=2. Nevertheless, its two counting functions agree for every m>=3, and its graph join with K_1 is enumeratively chromatic-choosable (Allred and Mudrock 2026, Theorems 11 and 29). Allred and Mudrock (2026, Conjecture 6) conjecture that every graph G has some complete graph K_p for which G v K_p is enumeratively chromatic-choosable.


See also

Choice Number, Chromatic Number, Chromatic Polynomial, List Color Function

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References

Allred, S. and Mudrock, J. A. "Enumerative Chromatic Choosability." Elec. J. Combin. 33, P4.6, 2026. https://doi.org/10.37236/14176.

Cite this as:

Weisstein, Eric W. "Enumerative Chromatic Choosability." From MathWorld--A Wolfram Resource. https://mathworld.wolfram.com/EnumerativeChromaticChoosability.html

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