Remove the Assumption D22 = 0
Suppose K(s) is a
controller for G with D22 set to zero. Then the controller for D22 ¹ 0 is K(I+ D22 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=- ex where e>0 is used, then
Hence the norm can be made arbitrarily small as e®0, but e=0 is not stabilizing.