Show that the Petersen graph is isomorphic to the complement of the triangular graph 
. The isomorphisms of a graph G to itself are called automorphisms; clearly, they form a group, the automorphism group of G. In this book we will not study automorphisms of graphs, except for some comments on Cayley graphs in Chapter 9; we refer the reader to [Yap86], [Har69], or [CaLi91]. However, we give an exercise concerning this topic.