中文摘要：

产业集聚是提高城市生产率的重要途径，而专业化和多样化则是集聚模式的两种选择。文章以城市规模作为门限变量，利用门限回归方法构建非线性面板数据模型，实证检验了不同城市规模区间内专业化和多样化对城市生产率的差异性影响。结果发现：专业化集聚对中小规模城市的生产率具有一定的促进作用，对较大规模的城市会在某种程度上阻碍城市生产率的提高；多样化集聚与小规模城市的生产率显著负相关，与中等规模城市的生产率不相关，但对较大规模城市的生产率具有十分显著的正向效应。其政策含义为，中小规模城市应集中优势资源发展重点产业，依靠专业化实现城市之间更为合理的分工协作；较大规模城市则需鼓励多种产业共同推进，通过多样化集聚改善城市生产率。

英文摘要：

Industrial agglomeration is an important way to improve ur- ban productivity, and specialization and diversification are two options for agglomeration mode. This paper takes city scale as the threshold variable, and employs the threshold regression method to establish a nonlinear panel data model. Then it empirically studies the effects of specialization and di- versification on the differences of urban productivity. It arrives at the fol- lowing conclusions： firstly, specialization agglomeration improves the pro- ductivity of small and medium-sized cities to a certain extent, but hinders the productivity improvement in large cities; secondly, diversification agglomer- ation has the significantly negative and positive correlation with the produc- tivity of small and large cities respectively, but has no relationship with the productivity of medium-sized cities. It provides the policy implications that small and medium-sized cities should focus on developing key industries, and achieve a more rational division of labor through specialization, while large cities r~eed to encourage a joint development of a variety of industries and promote urban productivity through diversification agglomeration.