Problems on pairs of trees and the four colour problem of planar graphs

Czumaj, A., & Gibbons, A. (1993). Problems on pairs of trees and the four colour problem of planar graphs. In Lecture Notes in Computer Science (including subseries Lecture Notes in Artificial Intelligence and Lecture Notes in Bioinformatics) (Vol. 700 LNCS, pp. 88–101). Springer Verlag. https://doi.org/10.1007/3-540-56939-1_64