http://www.sciencedirect.com/science?_ob=PublicationURL&_tockey=%23TOC%234861%232006%23998609998%23620760%23FLA%23&_auth=y&view=c&_acct=C000050221&_version=1&_urlVersion=0&_userid=10&_pubType=J&md5=48b885eb6e01bdd8b5000e9cbf975b6d