Abstract
Several constructions of 4-critical planar graphs are given. These provide answers to two questions of B. Grünbaum and give improved bounds for the maximum edge density of such graphs.
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Abbott, H.L., Zhou, B. The edge density of 4-critical planar graphs. Combinatorica 11, 185–189 (1991). https://doi.org/10.1007/BF01205071
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DOI: https://doi.org/10.1007/BF01205071