http://www.sciencedirect.com/science?_ob=MImg&_imagekey=B6V0C-4FNTG55-1-3G&_cdi=5643&_user=119223&_pii=S0020025505000654&_origin=search&_coverDate=08%2F03%2F2006&_sk=998239984&view=c&wchp=dGLzVzz-zSkWb&md5=924260f75f68e8cf10c3216eaae7056c&ie=/sdarticle.pd