A survey of some recent results in the stability and instability analyses of time-varying systems

After formulating the problem of stability and instability of linear and non-linear time varying feedback systems, we give a concise survey of a few known stability results for non-linear time invariant and time varying systems. This serves as motivation for the establishment of general frequency domain stability and instability criteria for time varying linear and non-linear feedback systems. All the details are omitted in an attempt to give an overall view. Finally some comments are made on the stability analysis of systems governed by partial differential equations