1. On 4-chromatic cubic graphs;Adelson-Velskij;Vopr. Kibernet. (Moscow),1974
2. The extension of a 3-coloring on planar graphs;Aksionov;Diskret. Analiz,1974
3. The reducibility of maps;Birkhoff;Am. J. Math.,1913
4. Le problème des quatre couleurs;Blanuša;Hrvatsko Prirodoslovno Društvo. Glas. Mat.-Fiz. Astr. Ser. II.,1946
5. A new proof of Grünbaum’s 3 color theorem;Borodin;Discrete Math.,1997