@article{1256049012,
author = {Appel, K. and Haken, W. and Koch, J.},
title = {Every planar map is four colorable. Part II: Reducibility},
journal = {Illinois J. Math.},
volume = {21},
number = {4},
year = {1977},
pages = { 491-567},
language = {en},
url = {http://dml.mathdoc.fr/item/1256049012}
}
Appel, K.; Haken, W.; Koch, J. Every planar map is four colorable. Part II: Reducibility. Illinois J. Math., Tome 21 (1977) no. 4, pp. 491-567. http://gdmltest.u-ga.fr/item/1256049012/