http://www.sciencedirect.com/science?_ob=MImg&_imagekey=B6VDY-4FD79R6-C-1&_cdi=5995&_user=119223&_pii=S0921800904003969&_origin=search&_coverDate=07%2F01%2F2005&_sk=999459998&view=c&wchp=dGLbVlb-zSkzV&md5=07c285ed560b3379908958bf3322302d&ie=/sdarticle.pdf