For the linear time invariant system
![\\bigl [\\matrix {y_{c}(s)\\cr y_{m}(s)}\\bigr ] = \\bigl [\\matrix{G(s)&H(s)\\cr M(s)&N(s)} \\bigr ] \\bigl [\\matrix {u_{c}(s)\\cr u_{r}(s)}\\bigr ]](/images/tex/3338.gif)
a necessary and sufficient condition based on the proper rational transfer function matrices

is given for the existence of a proper stabilizing controller

. The condition states that the McMillan degrees of

and
![\\bigl [\\matrix{G^{+}(s)&H^{+}(s)\\cr M{+}(s)&N{+}(s)}\\bigr ]](/images/tex/3342.gif)
must be equal, where

represents the unstable components of the partial fraction expansions of

.