Abstract
We consider several constructions of edge critical 4-chromatic graphs which can be written as the union of a bipartite graph and a matching. In particular we construct such a graph G with each of the following properties: G can be contracted to a given critical 4-chromatic graph; for each n ≥ 7, G has n vertices and three matching edges (it is also shown that such graphs must have at least \({{8n} \over 5}\) edges); G has arbitrary large girth.
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Chen, G., Erdős, P., Gyárfás, A. et al. A Class of Edge Critical 4-Chromatic Graphs. Graphs and Combinatorics 13, 139–146 (1997). https://doi.org/10.1007/BF03352991
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DOI: https://doi.org/10.1007/BF03352991