Stephen Prajna Ali Jadbabaie Safety Verification of Hybrid Systems Using Barrier Certificates. 477-492 http://springerlink.metapress.com/openurl.asp?genre=article&issn=0302-9743&volume=2993&spage=477 2004 conf/hybrid/2004 HSCC db/conf/hybrid/hscc2004.html#PrajnaJ04