Morgan's problem is still open
H. Herrera, J.F. Lafay · 1992
It is shown that the Morgan problem is not completely solved in two aspects. First, the Descusse, Lafay, and Malabre condition is necessary and sufficient only for a special class of systems. In general, it is only necessary, and second, the essential orders are not necessarily the minimal infinite structure obtained when decoupling a static decouplable system. A necessary and sufficient condition for statically decoupling a system without modifying the essential orders is presented for the case k=p-1 where p is the number of outputs and k is the rank at infinity of the proper part of the interactor.>