Remove the Assumption* D**22* = 0

Suppose *K(s)* is a
controller for *G* with *D**22* set to zero. Then the controller for *D**22* ¹ 0 is *K(I+ D**22* *K)**-1** .*

Relaxing A3 and A4

Complicated. Suppose that

which violates both A3 and A4 and corresponds to the robust stabilization of an integrator.
If the controller *u=- *e*x* where e>0 is used, then

Hence the norm can be made arbitrarily small as e®0, but e=0 is not stabilizing.