http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6T3H-3YF3WGW-7&_user=10&_rdoc=1&_fmt=&_orig=search&_sort=d&view=c&_acct=C000050221&_version=1&_urlVersion=0&_userid=10&md5=0e90e2facee81281438fb42469f7b659