http://www.sciencedirect.com.ezproxy.pvc.maricopa.edu/science/article/pii/S0065262808600105
http://www.sciencedirect.com.ezproxy.pvc.maricopa.edu/science/article/pii/B012227055X005745
http://www.sciencedirect.com.ezproxy.pvc.maricopa.edu/science/article/pii/S0065262808600658...