1. C. Berge, Graphs and Hypergraphs, North-Holland, Amsterdam, 1973, p. 188.
2. G. Chartrand, Introductory Graph Theory, Dover Publications, Mineola, NY, 1985, pp. 133–135.
3. H. Dudeney, Amusements in Mathematics, Dover Publications, Mineola, NY, 1970, p. 103.
4. W. W. Rouse Ball and H. S. M. Coxeter, Mathematical Recreations and Essays. University of Toronto Press, Toronto, 1974, pp. 175–185.