3 Appel K., Haken W. The solution of the four-color-map problem // Scientific American, October 1977. P. 108–121.

4 Saaty T. L., Kainen P. C. The Four-Color Problem: Assaults and Conquest. — McGraw-Hill, 1977.

5 Davis O. J., Hersh R. The Mathematical Experience. — Penguin, 1990.