Let G be a k-chromatic graph. G is called a Kempe graph if all k-colorings of G are Kempe equivalent. It is an unsolved and hard problem to characterize the properties of Kempe graphs with chromatic number ≥3. The Kempe equivalence of maximal planar graphs is addressed in this paper. Our contributions are as follows: (1) Observe and study a class of subgraphs, called 2-chromatic ears, which play a critical role in guaranteeing the Kempe equivalence between two 4-colorings; (2) Introduce and explore the properties of σ-characteristic graphs, which clearly characterize the relations of all 4-colorings of a graph; (3) Divide the Kempe equivalent classes of non-Kempe 4-chromatic maximal planar graphs into three classes, tree-type, cycle-type, and circular-cycle-type, and point out that all these three classes can exist simultaneously in the set of 4-colorings of one maximal planar graph; (4) Study the characteristics of Kempe maximal planar graphs, introduce a recursive domino method to construct such graphs, and propose two conjectures.
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