Monotone Control Systems
  Monotone systems constitute one of the most important classes of dynamical systems used in mathematical biology modeling.   The objective of this paper is to extend the notion of monotonicity to systems with inputs and outputs, a necessary first step in trying to understand interconnections, especially including feedback loops, built up out of monotone components.   Basic definitions and theorems are provided, as well as an application to the study of a model of one of the cell's most important subsystems. 
David Angeli and Eduardo D. Sontag