Exact and Positive Controllability of Boundary Control Systems
  Using the semigroup approach to abstract boundary control problems we characterize the space of all exactly reachable states. Moreover, we study the situation when the controls of the system are required to be positive. The abstract results are applied to flows in networks with static as well as dynamic boundary conditions. 
Klaus-Jochen Engel and Marjeta Kramar Fijav\v{z}
math.FA math.OC