  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
