Abstract :
A central problem in control theory is the design of feedback controllers so as to have certain outputs of a given plant to track prescribed reference trajectories. In any realistic scenario, this control goal has to be achieved in spite of a good number of phenomena which would cause the system to behave differently from that expected. These phenomena could be endogenous, for instance parameter variations, or exogenous, such as additional undesired inputs affecting the behaviour of the plant. For a time-invariant, finite-dimensional system, the problem in question can be formally cast as follows
