A positive linear system is a linear system in which state variables, control and measurements are always nonnegative. In many systems of practical importance, all the state variables are not available for measurements, although the entire state vector has to be known for control strategies for example. In this situation, one can construct an approximation of the full state vector based on available measurements. Such a device is known as an observer. In particular, this study determines

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... ral conditions on positive linear systems for the existence of convergent positive linear observers. Single output systems are first considered. The extension to multi output systems is then performed.