http://www.sciencedirect.com/science?_ob=MImg&_imagekey=B6T16-4RJKWWK-G-1&_cdi=4882&_user=119223&_pii=S0167527307020578&_origin=&_coverDate=04%2F03%2F2009&_sk=998669997&view=c&wchp=dGLbVlz-zSkWA&md5=666f65126d894e516da0de740ce9c62b&ie=/sdarticle.pdf