Example

Let *P* and *K* be two-by-two transfer matrices

Then

So the closed-loop system is not stable even though

has no zero in the closed right-half plane and
the number of unstable poles of *PK=n**k**+n**p**=1*. Hence, in general, *det(I+PK)* having no zeros in the closed right-half
plane does not necessarily imply *(I+PK)**-1*Î *RH**¥* .