http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6WB2-4MMWHDJ-4&_user=4420&_coverDate=12%2F23%2F2006&_rdoc=1&_fmt=&_orig=search&_sort=d&view=c&_acct=C000059607&_version=1&_urlVersion=0&_userid=4420&md5=e99a535441f194bbe5d479dd2e344766