Gaoyan Xie Zhe Dang Oscar H. Ibarra Pierluigi San Pietro Dense Counter Machines and Verification Problems. 93-105 http://springerlink.metapress.com/openurl.asp?genre=article&issn=0302-9743&volume=2725&spage=93 2003 conf/cav/2003 CAV db/conf/cav/cav2003.html#XieDIP03