http://www.sciencedirect.com/science?_ob=MImg&_imagekey=B6V21-4N7Y8CB-1-2N&_cdi=5689&_user=119223&_pii=S0005109807000271&_origin=&_coverDate=05%2F31%2F2007&_sk=999569994&view=c&wchp=dGLzVlz-zSkzS&md5=443a4c97fc2dc6c643992993e5670264&ie=/sdarticle.pdf