http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6VBF-521M6GJ-3&_user=1067472&_coverDate=01%2F25%2F2011&_rdoc=1&_fmt=high&_orig=search&_origin=search&_sort=d&_docanchor=&view=c&_acct=C000051251&_version=1&_urlVersion=0&_userid=1067472&md5=f50e61c06d4da577a6a0aa93001e4e07&searchtype=a