Appel, Kenneth; Haken, Wolfgang; Koch, John (1977), "Every Planar Map is Four Colorable. II. Reducibility", Illinois Journal of Mathematics, 21 (3): 504, doi:10.1215/ijm/1256049012, MR0543793, Of the 1834 configurations in 𝓤
doi.org
Appel, Kenneth; Haken, Wolfgang; Koch, John (1977), "Every Planar Map is Four Colorable. II. Reducibility", Illinois Journal of Mathematics, 21 (3): 504, doi:10.1215/ijm/1256049012, MR0543793, Of the 1834 configurations in 𝓤