正如我们在本章中看到的那样，各国中央银行并不是有目的地确定货币供给水平，而是设立一个短期

目标利率，并在这样的利率水平上随时向人们借贷他们所愿意持有的任何数量的货币。(当人们不是因为利率变化而是因为其他原因愿意持有更多货币的时候，货币供给因此得到扩张，当人们愿意持有更少货币时，货币供给就会紧缩.)

a.如果中央银行通过固定市场利率来制定货币政策，描述一这时有可能出现的问题。(首先，考虑价格灵活变动的情况，并且试问当中央银行以固定利率向人们提供他们所需的所有货币的时候，你是否会找到唯一的均衡价格水平。然后，考虑存在价格黏性的情况。)

b.根据公式R-R₀=a(P- P₀)，其中a为正常数，P₀为目标价格水平，如果中央银行在价格上升的时候提高利率，形式会发生变化吗？

c.假设中央银行的政策规则是R-R₀=a(P-P₀)+u，这里的u代表利率政策中的一种随机扰动。在教材中图14-12显示的超调模型中，描述了在一次未预期的随机因素u下降的情况下，经济是怎样调整到长期水平的，并且解释其中的原因。你可以将u的下降解释为中央银行削减利率，并且因此执行了一次货币扩张行动。请将这一.过程与教材中图14-13所描述的过程进行比较。

Typically central banks， rather than purposefully setting the level of the money supply， set a target level for a short-term interest rate by standing ready to lend or borrow whatever money people wish to hold at that interest rate. (When people need more money for some reason other than a change in the interest rate， the money supply therefore expands， and it contracts when they wish to hold less. In the United States， the“policy” interest rate that the Fed sets is called the federal funds rate， and it is the interest rate at which banks make overnight loans to each other.)

a. Describe the problems that might arise if a central bank sets monetary policy by holding the market interest rate constant. First， consider the flexible-price case， and ask yourself if you can find a unique equilibrium price level when the central bank simply gives people all the money they wish to hold at the pegged interest rate. Then consider the sticky-price case.)

b. Is the situation changed if the central bank raises the interest rate when prices are high， according to a formula such as R- R₀=a(P- P₀)， where a is a positive constant and Po a target price level？

c. Suppose the central bank's policy rule is R-R₀=a(P-P₀)+u， where u is a random movement in the policy

interest rate. In the overshooting model shown in Figure 14-12， describe how the economy would adjust to a permanent one-time unexpected fall in the random factor u， and why. You can interpret the fall in u as an interest rate cut by the central bank， and therefore as an expansionary monetary action. Compare your story with the one depicted in Figure14-13.