Regular Separability of Parikh Automata
  We investigate a subclass of languages recognized by vector addition systems, namely languages of nondeterministic Parikh automata. While the regularity problem (is the language of a given automaton regular?) is undecidable for this model, we show surprising decidability of the regular separability problem: given two Parikh automata, is there a regular language that contains one of them and is disjoint from the other? 
Lorenzo Clemente, Wojciech Czerwi\'nski, S{\l}awomir Lasota, Charles   Paperman
cs.FL