Kenneth A. Berman. Three-colourings of planar 4-valent maps. Journal of Combinatorial Theory, Series A, 30(1):82-88, 1981. [doi]
@article{Berman81:0,
title = {Three-colourings of planar 4-valent maps},
author = {Kenneth A. Berman},
year = {1981},
doi = {10.1016/0095-8956(81)90094-0},
url = {http://dx.doi.org/10.1016/0095-8956(81)90094-0},
researchr = {https://researchr.org/publication/Berman81%3A0},
cites = {0},
citedby = {0},
journal = {Journal of Combinatorial Theory, Series A},
volume = {30},
number = {1},
pages = {82-88},
}