Orna Kupferman Nir Piterman Lower Bounds on Witnesses for Nonemptiness of Universal Co-Büchi Automata. 182-196 2009 FOSSACS http://dx.doi.org/10.1007/978-3-642-00596-1_14 conf/fossacs/2009 db/conf/fossacs/fossacs2009.html#KupfermanP09