Stephane Gaubert Ricardo Katz Reachability and Invariance Problems in Max-plus Algebra. 15-22 2003 conf/posta/2003 POSTA http://springerlink.metapress.com/openurl.asp?genre=article&id=GVVWV0P0XD3V6U4X db/conf/posta/posta2003.html#GaubertK03