ABSTRACT
The concept of invariance is well known to be of utmost importance in the theory of dynamical systems. While this theory is basically concerned with flows generated by autonomous differential or difference equations, in recent years interest has been growing in dynamical systems which are exposed to external excitations such as deterministic or stochastic perturbations as well as controls. Since those systems are explicitly time dependent they are hard to be dealt with in the classical context of dynamical systems, and therefore it is necessary to develop time-variant versions of the fundamental concepts of dynamical systems theory. In this expository article we shed some light on the role invariance can play in the framework of time-variant discrete dynamics.
