http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6W9W-4SXRYFG-1&_user=99318&_coverDate=09%2F30%2F2008&_rdoc=1&_fmt=full&_orig=search&_cdi=6693&_sort=d&_docanchor=&view=c&_acct=C000007678&_version=1&_urlVersion=0&_userid=99318&md5=1d0083f1c47d534d798b8baddd8f09fc#sec2.5