Reducing complexity of tail-biting trellises
  It is shown that a trellis realization can be locally reduced if it is not state-trim, branch-trim, proper, observable, and controllable. These conditions are not sufficient for local irreducibility. Making use of notions that amount to "almost unobservability/uncontrollability", a necessary and sufficient criterion of local irreducibility for tail-biting trellises is presented. 
Heide Gluesing-Luerssen and G. David Forney, Jr
cs.IT cs.SY math.IT