Codes on Graphs: Observability, Controllability and Local Reducibility
  This paper investigates properties of realizations of linear or group codes on general graphs that lead to local reducibility.   Trimness and properness are dual properties of constraint codes. A linear or group realization with a constraint code that is not both trim and proper is locally reducible. A linear or group realization on a finite cycle-free graph is minimal if and only if every local constraint code is trim and proper.   A realization is called observable if there is a one-to-one correspondence between codewords and configurations, and controllable if it has independent constraints. A linear or group realization is observable if and only if its dual is controllable. A simple counting test for controllability is given. An unobservable or uncontrollable realization is locally reducible. Parity-check realizations are controllable if and only if they have independent parity checks. In an uncontrollable tail-biting trellis realization, the behavior partitions into disconnected subbehaviors, but this property does not hold for non-trellis realizations. On a general graph, the support of an unobservable configuration is a generalized cycle. 
G. David Forney Jr and Heide Gluesing-Luerssen
cs.IT cs.SY math.IT