The problem is to determine the optimal replacement policy ( N , T )? such that the long-run expected cost per unit time is minimized.

利用几何过程，以系统大修次数N和大修时间间隔T为更换策略，选择最优策略（ N ， T）使系统经长期运行单位时间的期望费用最小。

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Secretary-General Kofi Annan, including the creation of a peace-building commission to manage post-conflict aid and reconstruction as well as the replacement of the U.N.

CNN: U.S.: Reform U.N. before expanding Security Council