Patricia Bouyer Serge Haddad Pierre-Alain Reynier Undecidability Results for Timed Automata with Silent Transitions. 1-25 2009 92 Fundam. Inform. 1-2 http://dx.doi.org/10.3233/FI-2009-0063 db/journals/fuin/fuin92.html#BouyerHR09