Do this in two ways: (a) directly from the definition of the observability matrix, and (b) by duality, using Proposition 4.3. Proposition 5.2 Let A and T be nxn and C be pxn. If (C, A) is observable and T is nonsingular, then (T-¹AT, CT) is observable. That is, observability is invariant under linear coordinate transformations. Proof. The proof is left to Exercise 5.1.