We consider a MIMO linear time-invariant feedback system
1 
which is assumed to be

-stable. The plant

is subjected to an additive perturbation

which is proper but not necessarily stable. We prove that the perturbed system is

-stable if and only if
![\\Delta P[I + Q.\\Delta P]^{-1}](/images/tex/3392.gif)
is

-stable. (Here
