http://www.sciencedirect.com/science?_ob=MImg&_imagekey=B6V21-4KWTFJ9-2-C&_cdi=5689&_user=119223&_pii=S0005109806002810&_origin=&_coverDate=12%2F31%2F2006&_sk=999579987&view=c&wchp=dGLzVzb-zSkWb&md5=5ee973d5c0340363b0af97e3f633c683&ie=/sdarticle.pdf