H.-Q. Hu, J. Zhou and Z.-Z. Zheng, “Blue and Yellow Zone Logistics Network Nodes Optimization Research,” Shandong Social Science, Vol. 2, 2013, pp. 116-119.
 W. Zhou, B. Zhang, B. Zhang, et al., “Spatial Relationship between Dual-core Cities Based on Gravity Model— A Case of Chengdu and Chongqing City,” Resource Development & Market, Vol. 26, No. 11, 2010, pp. 82-85.
 W.-L. Lan, S.-P. Tan and Y.-Q. Ma, “An Analysis of Costs of Roads in different Grades,” Journal of Wuhan Institute of Shipbuilding Technology, Vol. 6, 2009, pp. 74-78.