A Note on the Complexity of Computing the Smallest Four-Coloring of Planar Graphs
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We show that computing the lexicographically first four-coloring for planar graphs is P^NP-hard. This result optimally improves upon a result of Khuller and Vazirani who prove this problem to be NP-hard, and conclude that it is not self-reducible in the sense of Schnorr, assuming P ≠ NP. We discuss this application to non-self-reducibility and provide a general related result.
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