When to call a linear system nonnegative

In this paper we will consider discrete time invariant linear systems that allow for an input-state-output representation with a finite dimensional state space, and that have a finite number of inputs and outputs. The basic issue in this paper is when to call these systems nonnegative. An important concept in this respect is that of the most powerful unfalsified model. (C) 1998 Elsevier Science Inc. All rights reserved.