http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6VF0-49KSRCX-1&_user=10&_handle=W-WA-A-A-AE-MsSAYZW-UUA-AUDBCBEVAB-ADVVVBWYV-AE-U&_fmt=full&_coverDate=01%2F31%2F2004&_rdoc=16&_orig=browse&_srch=%23toc%235996%232004%23999599998%23472776!&_cdi=5996&view=c&_acct=C000050221&_version=1&_urlVersion=0&_userid=10&md5=161dc05f2ffacce616358f9993887984