TY  - RPRT
TI  - Solution of the heawood map-coloring problem.
AU  - G Ringel
AU  - J W Youngs
PY  - 1968
DO  - 10.1073/pnas.60.2.438
UR  - https://pubmed.ncbi.nlm.nih.gov/16591648/
ID  - 16591648
ER  -