http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6VBC-4RWBWNX-1&_user=3846826&_coverDate=04%2F30%2F2008&_rdoc=13&_fmt=high&_orig=browse&_srch=doc-info(%23toc%235923%232008%23999729992%23688002%23FLA%23display%23Volume)&_cdi=5923&_sort=d&_docanchor=&_ct=15&_acct=C000061542&_version=1&_urlVersion=0&_userid=3846826&md5=ee6a30ecc2dcd35d476c9a22ceb7311e