http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6T1W-4JW7FH2-2&_user=10&_coverDate=09%2F30%2F2006&_alid=1184190086&_rdoc=8&_fmt=high&_orig=search&_cdi=4901&_sort=r&_docanchor=&view=c&_ct=355&_acct=C000050221&_version=1&_urlVersion=0&_userid=10&md5=242cd96124ba1c557eef84cd21619033 http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6TG8-4VP668M-1&_user=10&_coverDate=05%2F08%2F2009&_alid=1184190086&_rdoc=67&_fmt=high&_orig=search&_cdi=5248&_sort=r&_docanchor=&view=c&_ct=355&_acct=C000050221&_version=1&_urlVersion=0&_userid=10&md5=6f18b8960f2bae11083599bb6f1a38b4