In this section, we consider the state-feedback -control problem for the affine nonlinear system Σa in the presence of unmodelled dynamics and/or parameter variations. This problem has been considered in many references [6, 7, 209, 223, 245, 261, 265, 284]. The approach presented here is based on [6, 7] and is known as guaranteed-cost control. It is an extension of quadratic-stabilization, and was first developed by Chang  and later popularized by Petersen [226, 227, 235]. For this purpose, the system is represented by the model:
where all the variables ...