A new representation of SISO systems for studying approximated models

A non linear matrix equation not having previously appeared in the control literature is introduced in this paper. It is shown that a particular solution P- of the previous equation contains for SISO systems the same type of information carried in the closed loop characteristic values. In addition it also contains equivalent information to the so-defined closed loop system "signature". The importance of the P- matrix is outlined as regards the "model reduction problem".