5Friedman D, Fung K C. International trade and the internal or- ganization of firms: an evolutionary approach [J]. Journal of In- ternational Economics, 1996, (41): 113 - 137.
6Krugman P. Increasing Returns and Economic Geography[J]. Journal of Political Economy, 1991 , (3): 483-499.
7Bischi G I, Gardini L, Kopel M. Spillover effects and the evo- lution of firm clusters [J ]. Journal of Economic Behavior & Or- ganization, 2003, (50): 47 - 75.
