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On 3-colorable planar graphs without cycles of four lengths. (English) Zbl 1185.05061
Grötzsch’s Theorem states that planar graphs without 3-cycles are (vertex) 3-colorable. R. Steinberg conjectured that every planar graph without 4- or 5-cycles is 3-colorable, and P. Erdős asked if there is an integer $$k\geq5$$ such that a planar graph without $$i$$-cycles for $$i= 4,5,\dots,k$$ is 3-colorable [see R. Steinberg, “The state of the three color problem”, in Quo vadis, graph theory? A source book for challenges and directions. Ann. Discrete Math. 55, 211–248 (1993; Zbl 0791.05044)]. The best-known result answering Erdős’s question is that $$k\leq7$$, proven by O. V. Borodin et al. [O.V. Borodin, A.N. Glebov, A. Raspaud, and M.R. Salavatipour, “Planar graphs without cycles of length from 4 to 7 are 3-colorable”, J. Comb. Theory, Ser. B 93, No. 2, 303–311 (2005; Zbl 1056.05052)]. Thus an interesting question is that of determining small sets of small integers whose absence as cycle lengths ensures 3-colorability. For sets of four omitted cycle-lengths, it is known that lacking $$\{4,5,6,9\}$$-cycles [L. Zhang and B. Wu, “A note on 3-choosability of planar graphs without certain cycles”, Discrete Math. 297, No. 1-3, 206–209 (2005; Zbl 1070.05046)], or $$\{4,5,7,9\}$$-cycles [W. F. Wang and Y. Wang, J. Liaoning Univ. Nat. Sci. 32, No. 4, 302–305 (2005)] or $$\{4,6,7,9\}$$-cycles [M. Chen, A. Raspaud, and W. F. Wang, “Three-coloring planar graphs without short cycles”, Inf. Process. Lett. 101, No. 3, 134–138 (2007; Zbl 1185.05057)] or $$\{4,5,6,8\}$$-cycles [W. F. Wang and M. Chen, “On 3-colorable planar graphs without prescribed cycles”, Discrete Math. 307, No. 22, 2820–2825 (2007; Zbl 1128.05025)] ensures 3-colorability. In this paper the authors prove that a planar graph without $$\{4,6,7,8\}$$-cycles is 3-colorable. They do so by proving that every 3-coloring of the vertices of a face of length 5, 9, 10, 11 or 12 extends to a 3-coloring of the whole graph, and by using a self-contained discharging argument. Beginning with work of C. Thomassen [J. Comb. Theory, Ser. B 64, No. 1, 101–107 (1995; Zbl 0822.05029)], similar results are being obtained on 3-choosability, that is, when lists of size at least three are assigned to each vertex and a (vertex) coloring is sought in which each vertex receives a color from its list [see, e.g., L. Zhang and B. Wu, [loc. cit.]; Graph Theory Notes N. Y. 46, 27–30 (2004)].
Reviewed by Joan Hutchinson (M.R. 2008g:05077)

##### MSC:
 05C15 Coloring of graphs and hypergraphs 05C10 Planar graphs; geometric and topological aspects of graph theory 05C38 Paths and cycles
##### Keywords:
planar graph; coloring; cycle; length
Full Text:
##### References:
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