List colourings of planar graphs
Discrete Mathematics
Every planar graph is 5-choosable
Journal of Combinatorial Theory Series B
Algorithmic complexity of list colorings
Discrete Applied Mathematics
3-list-coloring planar graphs of girth 5
Journal of Combinatorial Theory Series B
A not 3-choosable planar graph without 3-cycles
Discrete Mathematics
Information Processing Letters
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A graph is $k$-choosable if it can be colored whenever every vertex has a list of at least $k$ available colors. We prove that if a triangle-free planar graph is not 3-choosable, then it contains a 4-cycle that intersects another 4- or 5-cycle in exactly one edge. This strengthens Thomassen's result [C. Thomassen, J. Combin. Theory Ser. B, 64 (1995), pp. 101-107] that every planar graph of girth at least 5 is 3-choosable. In addition, this implies that every triangle-free planar graph without 6- and 7-cycles is 3-choosable.