Stability of a nonlinear time-invariant feedback system under almost constant inputs

We consider a multiple-input multiple-output feedback system consisting of a linear time-invariant subsystem and a memoryless time-invariant nonlinearity. The linear subsystem is represented by its impulse response which includes a unit step, i.e. an integrator. The nonlinearity is not required to be of the noninteracting type nor to be bounded away from zero by some sector condition. It is shown that for any ''almost constant'' input, the error e"2 is bounded and goes to zero as t -> ~.