This paper develops a unified imperfectly competitive macroeconomic model, and uses it to analyze optimal fiscal policies in the presence of market imperfections. The salient feature of the model is that it is able to deal with three distinct types of market structure, including constant monopoly firms, endogenous monopoly firms and endogenous overhead costs. Several findings emerge from the analysis. First, the extent of production externalities will vary under the three distinct market structures, and in turn lead the government to implement different labor and capital tax policies to correct the different extents of the distortions. Second, the optimal ratio of government expenditure is determined solely by the extent of productive government spending if the number of firms is constant, while the optimal ratio is related to the internal increasing returns to scale and production specialization if the number of firms is determined endogenously. Finally, free entry in the competitive equilibrium may result in over entry or under entry, depending upon the relative degree between monopoly power and production specialization.

本文建立了一个统一的不完全竞争宏观经济模型，并用它来分析存在市场不完善情况下的最优财政政策。该模型的显着特征是它能够处理三种不同类型的市场结构，包括不变垄断企业，内生垄断企业和内生间接费用。分析得出了一些发现。首先，在三个不同的市场结构下，生产外部性的程度将有所不同，进而导致政府实施不同的劳动税和资本税政策，以纠正不同程度的扭曲。其次，如果公司数量不变，则政府支出的最佳比例仅取决于生产性政府支出的程度，如果企业数量是内生决定的，则最佳比率与内部规模收益和生产专业化程度有关。最后，取决于垄断权与生产专业化之间的相对程度，自由进入竞争均衡可能导致过度进入或进入不足。