Time

All planning problems involve a sequence of decisions that must be applied over time. Time might be explicitly modeled, as in a problem such as driving a car as quickly as possible through an obstacle course. Alternatively, time may be implicit, by simply reflecting the fact that actions must follow in succession, as in the case of solving the Rubik's cube. The particular time is unimportant, but the proper sequence must be maintained. Another example of implicit time is a solution to the Piano Mover's Problem; the solution to moving the piano may be converted into an animation over time, but the particular speed is not specified in the plan. As in the case of state spaces, time may be either discrete or continuous. In the latter case, imagine that a continuum of decisions is being made by a plan.