Non-Existence of Stabilizing Policies for the Critical Push-Pull Network   and Generalizations
  The push-pull queueing network is a simple example in which servers either serve jobs or generate new arrivals. It was previously conjectured that there is no policy that makes the network positive recurrent (stable) in the critical case. We settle this conjecture and devise a general sufficient condition for non-stabilizability of queueing networks which is based on a linear martingale and further applies to generalizations of the push-pull network. 
Yoni Nazarathy, Leonardo Rojas-Nandayapa and Thomas S. Salisbury
math.PR