Baxter's four-coloring constant for a triangular lattice is given by
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(1)
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(2)
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(3)
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(OEIS A142954).
Weisstein, Eric W. "Baxter's Four-Coloring Constant." From MathWorld--A Wolfram Resource. https://mathworld.wolfram.com/BaxtersFour-ColoringConstant.html