On the robust stability analysis of multivariable systems

The authors consider the robust stability of a multivariable system with both structured real parametric and unstructured norm-bounded uncertainties. It is shown that if the real parametric uncertainty enters only linearly and independently into the numerator matrix and the denominator characteristic polynomial, then the robust stability can be tested exactly. It is also shown that this in fact gives a way to compute a class of mixed real and complex structured values