Together, the initial state, actions, and transition model implicitly define the state space
of the problem—the set of all states reachable from the initial state by any sequence
of actions. The state space forms a directed network or graph in which the nodes
are states and the links between nodes are actions.
(The map of Romania shown in Figure 3.2 can be interpreted as a state-space graph if we view each road as standing
for two driving actions, one in each direction.)
A path in the state space is a sequence of states connected by a sequence of actions.