http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6WSS-4SY5JHT-6&_user=10&_rdoc=1&_fmt=&_orig=search&_sort=d&view=c&_acct=C000050221&_version=1&_urlVersion=0&_userid=10&md5=2997d273d99c96d5a1c22d7de7a906ba