Title: Selection of variables for stabilizing control using pole vectors
Authors: Kjetil Havre and Sigurd Skogestad
Published in: IEEE Trans. Autom. Control, Vol. 48, No. 8, August 2003, 1393-1398.

Abstract:
For a linear multivariable plant, it is known from earlier work that 
the easy computable pole vectors provide useful information about in 
which input channel (actuator) a given mode is controllable and in which
output channel (sensor) it is observable. In this paper we provide a 
rigorous theoretical basis for the use of pole vectors, by providing 
a link to previous results on performance limitations for unstable plants.