请用英语描述国际贸易理论有哪些?

【International Trade Theories】【国际贸易的各种理论】【Adam Smith's model】Adam Smith displays trade taking place on the basis of countries exercising absolute advantage over one another.【Ricardian model】The Ricardian model focuses on comparative advantage, which arises e to differences in technology or natural resources. The Ricardian model does not directly consider factor endowments, such as the relative amounts of labor and capital within a country.【Heckscher–Ohlin model】In the early 1900s a theory of international trade was developed by two Swedish economists, Eli Heckscher and Bertil Ohlin. This theory has subsequently been known as the Heckscher–Ohlin model (H–O model). The results of the H–O model are that countries will proce and export goods that require resources (factors) which are relatively abundant and import goods that require resources which are in relative short supply.【New Trade Theory】New Trade Theory tries to explain empirical elements of trade that comparative advantage-based models above have difficulty with. These include the fact that most trade is between countries with similar factor endowment and proctivity levels, and the large amount of multinational proction (i.e., foreign direct investment) that exists. Gravity model：The Gravity model of trade presents a more empirical analysis of trading patterns. The gravity model, in its basic form, predicts trade based on the distance between countries and the interaction of the countries' economic sizes. The model mimics the Newtonian law of gravity which also considers distance and physical size between two objects. The model has been proven to be empirically strong through econometric analysis.【Ricardian theory of international trade (modern development)】The Ricardian theory of comparative advantage became a basic constituent of neoclassical trade theory. Any undergraate course in trade theory includes a presentation of Ricardo's example of a two-commodity, two-country model. A common representation of this model is made using an Edgeworth Box.【Neo-Ricardian trade theory】The merit of neo-Ricardian trade theory is that input goods are explicitly included. This is in accordance with Sraffa's idea that any commodity is a proct made by means of commodities. The limitation of their theory is that the analysis is restricted to small-country cases.【Ricardo-Sraffa trade theory】Economist John S. Chipman observed in his survey that McKenzie stumbled upon the questions of intermediate procts and postulated that "introction of trade in intermediate proct necessitates a fundamental alteration in classical analysis".It took many years until Shiozawa succeeded in removing this deficiency.The Ricardian trade theory was now constructed in a form to include intermediate input trade for the most general case of many countries and many goods. Chipman called this the Ricardo-Sraffa trade theory.【International proction fragmentation trade theory】In his chapter entitled Li & Fung, Ltd.: An agent of global proction (2001), Cheng used Li & Fung Ltd as a case study in the international proction fragmentation trade theory through which procers in different countries are allocated a specialized slice or segment of the value chain of the global proction. Allocations are determined based on "technical feasibility" and the ability to keep the lowest final price possible for each proct.