•Let
be rcf and lcf, respectively. Then the following conditions are
equivalent:

1. The feedback system is
internally stable.

2. is invertible in *RH**¥* 3. is invertible in *RH**¥*

4. is invertible in *RH**¥* 5. is invertible in *RH**¥*

•Let be a stabilizable and
detectable realization, and let *F *and *L* be such that *A+BF* and *A+LC *are both stable.

Define

Then

Continued