A note on "On the generalized Nyquist stability criterion"

An alternative [2] generalized Nyquist stability criterion is proposed for L
2
systems. It is shown that the properties of a system transfer function matrix
\hat{G}(s)
associated with its invertibility in the restricted space of transfer function matrices can be deduced from simple encirclement conditions in the complex plane involving the loci of the eigenvalues of
G
.