http://www.sciencedirect.com/science?_ob=MImg&_imagekey=B6V81-4VP12BP-1-9&_cdi=5857&_user=10&_orig=browse&_coverDate=03%2F31%2F2009&_sk=999669998&view=c&wchp=dGLbVlb-zSkzV&md5=c203bd5300c681cf51b64f700c6eb7c6&ie=/sdarticle.pdf