Much of Chapter 3 is devoted to walks with some special property. In particular, we
discuss graphs containing walks that include every edge or every vertex exactly once,
ending at the initial vertex; such graphs are called Eulerian and Hamiltonian graphs,
respectively. For example, the graph in Figs 1.3-1.5 is Hamiltonian; a suitable walk is
F - ^ g ~ ) i ? - ) ^ r - ) P . I t i s not Eulerian, since any walk that includes each edge
exactly once (such asP-^Q-^R-^S-^T-^P-^S-^Q-^T) must end at a vertex
different from the initial one.