Necessary Condition for Observer-Based Chaos Synchronization

We consider observer-based synchronization of chaotic systems. In this scheme, for a given chaotic drive system, response system is chosen in observer form. We show by examples that many response systems proposed in the literature are of this form. We give a necessary condition for synchronization and a selection criterion for appropriate synchronization signal in this case. We apply this idea to synchronization of well-known hyperchaotic R\"ossler system.