隨著地球的汙染越發嚴重，許多廠商也開始自發性的關心環境。傳統上政府需要以課徵汙染稅的方式將環境外部性內部化，但如今廠商會自願將其內部化。因此本文欲研究若存在此情況，並在有著外人直接投資的混合雙占市場中，廠商的生產決策會有何不同，政府制定最適污染稅時又會有何差異。我們知道，政府以社會福利極大化為目標訂立最適污染稅；公營廠商以社會福利極大化為目標進行生產；民營廠商以利潤極大化為目標進行生產。本文探討市場上僅有一家本國公營廠商及一家外國民營廠商，以及加入一家本國民營廠商兩種情況。運作模式為政府先訂立污染稅，各廠商在依此稅率進行生產。而本文以逆推法求出此兩階段賽局。
在本國民營廠商尚未加入市場時，我們發現本國公營廠商生產成本、單位汙染排放量及環境污染提高會使污染稅上升；外國民營廠商生產成本、單位汙染排放量及關心環境程度提高會使最污染稅下降。我們發現若本國民營廠商加入市場後，本國民營廠商關心環境程度提升時並不影響污染稅；外國民營廠商生產成本及關心環境程度提高會使污染稅下降；本國公營廠商生產成本、本國民營廠商生產成本、三廠商的單位汙染排放量及環境損害提高時會使污染稅變動方向不定。

As the pollution of the earth becomes more and more serious, many manufacturers have also begun to take care of the environment spontaneously. Traditionally, governments needed to internalize environmental externalities by levying pollution taxes, but manufacturers are now willing to internalize them voluntarily. Therefore, this paper intends to study if such a situation exists, and in a mixed duopoly market with foreign direct investment, how the production decisions of manufacturers will be different, and how the government will set the optimal pollution tax. We know that the government sets the optimal pollution tax with the goal of maximizing social welfare; public cooperation produce with the goal of maximizing social welfare; private cooperation produce with the goal of maximizing profits. This paper discusses two situations in which there is only one domestic public firm and one foreign private firm in the market, and one domestic private firm is added. The mode of operation is that the government first sets the optimal pollution tax, and each manufacturer conducts production according to this tax rate. In this paper, the backward pass is used to find the two-stage game.
Before domestic private manufacturers have joined the market, we found that the increase in the production cost of domestic public manufacturers, unit pollution emissions of domestic public manufacturers and environmental pollution will increase the pollution tax; the production cost of foreign private manufacturers, the unit pollution emissions of foreign private manufacturers and the environmental concern of foreign private manufacturers will decrease the pollution tax. We also find that if domestic private manufacturers join the market, the pollution tax will not be affected when the domestic private manufacturers are more concerned about the environment; the production cost and the environmental concern of foreign private manufacturers will decrease the pollution tax; When the production cost of domestic public manufacturers, the production cost of domestic private manufacturers, the unit pollution emissions of the three manufacturers and the environmental damage increase, the direction of the pollution tax changes will be uncertain.

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