1Browning H, Singelman J. The emergence oi a service society: IAemograpnlc ana soclo~og~ca~ a~v~ 0, sectorial transformation of the labor force[A]. Springfield V A. National technical information service[C]. VA: USASpringfield, 1975.
2Helpman E. The size of regions I- A]. Pines D, Sadka E, Zilcha I. Topics in public economics[C]. Cambridge: Cambridge University Press, 1998.
3Hill T P. On goods and servicesEJ]. Review of Income and Wealth, 1977, 23(4): 315-338.
4Krugman P. Increasing returns and economic geography[J]. Journal of Political Economy, 1991, 99(3) : 483-499.