Note that a dominant strategy equilibrium is a special case of a Nash
equilibrium.
In the advertising game of Table 13.2, there is a single Nash equilibrium—both
firms advertise. In general, a game need not have a single Nash equilibrium.
Sometimes there is no Nash equilibrium, and sometimes there are several (i.e.,
several sets of strategies are stable and self-enforcing). A few more examples
will help to clarify this.