因学校未1995前的sciencedirect全文数据库，向大家求文献一篇：Abdel-RahmanHesham M. and Fujita Masahisa.Specialization andDiversification in a System of Cities, Journal ofUrban Economics,Volume33, Issue 2, March 1993, Pages 189-222.　5论坛币略表谢意！

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