http://www.sciencedirect.com/science?_ob=MImg&_imagekey=B6V7N-4H3JJD7-6-9&_cdi=5847&_user=119223&_pii=S0145213405001870&_origin=search&_coverDate=09%2F30%2F2005&_sk=999709990&view=c&wchp=dGLzVtz-zSkzS&md5=ee326fe544c2065696445be641a7c5e0&ie=/sdarticle.pdf