Johan Bengtsson Wang Yi On Clock Difference Constraints and Termination in Reachability Analysis of Timed Automata. 491-503 http://springerlink.metapress.com/openurl.asp?genre=article&issn=0302-9743&volume=2885&spage=491 2003 conf/icfem/2003 ICFEM db/conf/icfem/icfem2003.html#BengtssonY03