See:
http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6TC3-4NKJ1NF-1&_user=10&_rdoc=1&_fmt=&_orig=search&_sort=d&view=c&_acct=C000050221&_version=1&_urlVersion=0&_userid=10&md5=b83e587d5c6f98f4be02e1b165ce78d6
Link: www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6TC3-4NKJ1NF-1&_user=10&_rdoc=1&_fmt=&_orig=search&_sort=d&view=c&_acct=C000050221&_version=1&_urlVersion=0&_userid=10&md5=b83e587d5c6f98f4be02e1b165ce78d6