主要考虑外平面图，系列平行图和平面欧拉图这三类特殊的平面图。
In this paper we prove that the problem is polynomial solvable on several special classes of graphs, such as outerplanar graphs, series-parallel graphs and Eulerian planar graphs.
同时还确定了所有系列-平行图的边色数。
The chromatic index of any series parallel graph is also determined.
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