Joachim Apel Passive Complete Orthonomic Systems of PDEs and Involutive Bases of Polynomial Modules. 88-107 http://springerlink.metapress.com/openurl.asp?genre=article&issn=0302-9743&volume=2630&spage=88 2001 conf/snsc/2001 SNSC db/conf/snsc/snsc2001.html#Apel01