Sufficient Conditions on the Existence of Switching Observers for   Nonlinear Time-Varying Systems
  We derive sufficient conditions for the solvability of the observer design problem for a wide class of nonlinear time-varying systems, including those having triangular structure. We establish that, under weaker assumptions than those imposed in the existing works in the literature, it is possible to construct a switching sequence of time-varying noncausal dynamics, exhibiting the state determination of our system. 
D. Boskos and J. Tsinias
math.OC cs.SY