http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6TVN-4FY9J54-3&_user=10&_rdoc=1&_fmt=&_orig=search&_sort=d&_docanchor=&view=c&_searchStrId=1069679175&_rerunOrigin=google&_acct=C000050221&_version=1&_urlVersion=0&_userid=10&md5=3c438b6babd5afad8329378c2067125f