1. J. A. Bondy and U. S. R. Murty. 5th Ed. Graph Theory with Application (North Holland, Boston New York, 1982), pp. 1–3.
2. D. A. Marcus, Graph Theory: A Problem Oriented Approach (The Mathematical American Society, Washington DC, 2008), pp. 11.
3. J. J. Rotman, Advanced Modern Algebra (American Mathematical Society, Washington, 2010), pp. 99.
4. F. M. Goodman, Algebra, Abstract and Concrete (SemiSimple Press, Lowa City, 2006), pp. 242–243.
5. S. O. Kim, Comm. Korean. Math. Soc. 16, 205–210 (2001).