Given S an (A;B)-invariant subspace, we prove that the set of friend feedbacks is a (nm¡md+dq)-dimensional
linear variety, which can be considered as the direct sum of the feedbacks of the restriction to S and the co-restriction to S?.
In particular, if (A;B) is controllable and S is a controllability subspace, both pole assignments are simultaneously possible
by means of a convenient friend feedback.
Compta, A.; Ferrer, J.; Peña, M. On the effect of friend feedbacks. "AIP Conference proceedings", 2009, vol. 1168, p. 564-567.