Before revision: The first instance of such a principle is in the setting of deterministic linear systems, namely that if a stable observer and a stable state feedback are designed for a linear time-invariant system , then the combined observer and feedback is stable.
After revision: (LTI system hereafter)The first instance of such a principle is in the setting of deterministic linear systems, namely that if a stable observer and a stable state feedback are designed for a linear time-invariant system , then the combined observer and feedback is stable.