Observer design for nonlinear triangular systems with unobservable   linearization
  The paper deals with the observer design problem for a wide class of triangular time-varying nonlinear systems, with unobservable linearization. Sufficient conditions are derived for the existence of a Luenberger-type observer, when it is a priori known that the initial state of the system belongs to a given nonempty bounded subset of the state space. For the general case, the state estimation is exhibited by means of a switching sequence of time-varying dynamics 
D. Boskos, J. Tsinias
math.OC cs.SY