The Importance of Being Positive: Admissible Dynamics for Positive Systems

Positive linear systems display peculiar dynamics due to the positivity constraints on input, state and output variables. In this paper we review such peculiarities for externally and internally positive linear systems. The properties of externally positive systems are shown in terms of poles and zeros location and input-output response, and those of internally positive systems in terms of eigenvalues location. Open problems are also presented. The presentation style of this paper is very informal, aiming to convey to the reader just a taste of the "importance of being positive".