On a robust nonlinear servomechanism problem

A notion of k^th-order robust control for a general nonlinear servomechanism problem is introduced. Necessary and sufficient conditions are given for the existence of a k^th -order robust control law such that, regardless of small parameter perturbations of the plant and control law, the steady state error of the closed-loop system is guaranteed to be of (k+1 )^th -order in the exogenous signal