Tracking. State tracking is driving a solution x ( t ) to the prespecified function of time x ∗ ∗ ( t ) . Similarly, output tracking is driving the output y ( t ) to the desired output function y ∗ ∗ ( t ) . The problem is more complex if the desired equilibrium x ∗ ∗ or trajectory x ∗ ∗ ( t ) is unstable in the absence of control action. For example, a typical problem of chaos control can be formulated as tracking an unstable periodic solution (orbit). The key feature of the control problems for physical systems is that the goal should be achieved by means of sufficiently small control. A limit case is stabilization of a system by an arbitrarily small control. The solvability of this task is not obvious if the trajectory x ∗ ∗ ( t ) is unstable, for example in the case of chaotic systems. See.