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- Havel給出兩個(gè)反例,如果平面圖含有4-圈或有5-圈是不可3-可著(zhù)色的,因此4-圈和5-圈在證明平面圖是3-可著(zhù)色時(shí)必須排除.In Havel [2] (1969, Journal of Combinatorial Theory, 7:184~186), Havel provided that both 4-cycles and 5-cycles must be excluded to ensure that the plane graphs are 3-colorable.
