Problems on Pairs of Trees and the Four Colour Problem of Planar Graphs.

Artur Czumaj Alan Gibbons Problems on Pairs of Trees and the Four Colour Problem of Planar Graphs. 88-101 1993 conf/icalp/1993 ICALP db/conf/icalp/icalp93.html#CzumajG93 http://dx.doi.org/10.1007/3-540-56939-1_64