理想状态下的市场能够达到高效生产：将总剩余(total surplus)最大化。 但是在现实中的市场并非完美。在本课程中，我们将探索一系列市场存在着缺陷的情形并且理解为什么会出现这些缺陷；同时，我们会了解一些改进措施，比如反托拉斯(antitrust)政策以及其他一些政府的干预措施。 我们会选取日常中的例子——从我们每天购买和享用的货物和服务开始。我们也会在每周的练习里把理论运用到时下大事和政策研讨中。 所有的这些都会使你成为一个可以理解、分析和评价市场结果的，有学识的，有批判精神的思考者。

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From the course by 宾夕法尼亚大学

微观经济学：当市场“失灵”时

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理想状态下的市场能够达到高效生产：将总剩余(total surplus)最大化。 但是在现实中的市场并非完美。在本课程中，我们将探索一系列市场存在着缺陷的情形并且理解为什么会出现这些缺陷；同时，我们会了解一些改进措施，比如反托拉斯(antitrust)政策以及其他一些政府的干预措施。 我们会选取日常中的例子——从我们每天购买和享用的货物和服务开始。我们也会在每周的练习里把理论运用到时下大事和政策研讨中。 所有的这些都会使你成为一个可以理解、分析和评价市场结果的，有学识的，有批判精神的思考者。

From the lesson

Costs and Profits + Perfect Competition

In the first part of the course we learnt that if we allow market forces to work we reach an efficient outcome: the maximum benefit that can be generated by a market. The second part of the course explores cases where the markets fail to accomplish our goals. This week sets up the benchmark case of the perfectly competitive market: a model we will modify in the next few weeks. We define Perfect Competition, learn to model it graphically and discuss some key results in terms of long run profits and implications for efficiency.

- Rebecca SteinSenior Lecturer

Economics

[MUSIC]

So we've seen so far that when the firm is making positive profits or

when price is greater than average total cost, we'll have entry into the industry.

And when the firm's making negative profits or

when price is less than average total cost, we'll have exit.

That means that, in the long run, there's only one stable point, and

that one is when price is exactly equal to the average total cost,

because that is when profits are equal to zero.

There's no incentive for firms to enter, and no incentive for firms to exit.

So I want to point one thing out to you.

In my graphs, I said that the long run will be at this price here.

Not only as this price equal to the average total cost, but

this is the case where the price is equal to the minimum of the average total cost.

So let's just prove that, that must be correct.

We have to put together two equations.

The first equation is the profit maximizing equation which implies

that the firm is always going to produce where price is equal to marginal cost.

Again, that comes from profit maximization.

The second equation is that in the long run, profits are going to be

equal to zero, which means that price is equal to the average total cost.

So if we take these two claims together, price is equal to marginal cost and

price is equal to the average total cost,

together this implies that it must the case that in the long run,

the firm is producing what a marginal cost is equal to the average total cost.

And is only one spot where the margin intersects the average total cost and

that is at the minimum.

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