Given S an (A,B)-invariant subspace, we prove that the set of friend feedbacks is a 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, when
the natural controllability hypothesis hold, both pole assignments are simultaneously possible bymeans of a convenient
friend feedback. Copyright
Compta, A.; Ferrer, J.; Peña, M. On the effect of friend feedbacks. "Mathematical methods in the applied sciences", 2011, vol. 34, núm. 3, p. 309-314.