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=nk+np=1. Hence, in general, det(I+PK) having no zeros in the closed right-half
plane does not necessarily imply (I+PK)-1Î RH¥ .