http://www.sciencedirect.com/science?_ob=MImg&_imagekey=B6V0C-4MFCKGY-6-C&_cdi=5643&_user=119223&_pii=S0020025506003318&_origin=search&_coverDate=04%2F01%2F2007&_sk=998229992&view=c&wchp=dGLbVzz-zSkzk&md5=f3f23082462a5c70176c72eb959858fd&ie=/sdarticle.pdf