Inspired by the recent Einstein 'Hat' tiling. Is Appel & Haken's proof still applicable to an infinite aperiodic graph ? Such a graph with 1 region less still remains an infinite graph, right? How to apply the reduction principle then?
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8This follows from the De_Bruijn-Erdős theorem, so infinite planar graphs are 4-colorable. – LeechLattice Mar 31 '23 at 12:01