Pieter Collins Jan H. van Schuppen Observability of Piecewise-Affine Hybrid Systems. 265-279 http://springerlink.metapress.com/openurl.asp?genre=article&issn=0302-9743&volume=2993&spage=265 2004 conf/hybrid/2004 HSCC db/conf/hybrid/hscc2004.html#CollinsS04