Given an odd prime , put the numbers into pairs such that each number appears exactly once. Each pair has a cost of . For example, if the pair has a cost of .
The total cost of a pairing is the sum of the costs of its pairs. We say that such pairing is optimal if its total cost is minimal for that .
For example, if , then there is a unique optimal pairing: , with total cost of .
The cost product of a pairing is the product of the costs of its pairs. For example, the cost product of the optimal pairing for is .
It turns out that all optimal pairings for have the same cost product.
Find the value of this product.
现给定一奇质数 ,我们将 这些数分成 个数对,每个数恰好出现一次。每个数对 有费用 。例如 时,数对 的费用即为 。
我们称一个配对方案的总花费为其所有数对的费用之和。如果对于某个 ,有一个配对方案,使得它的总花费在所有配对方案中最小,则我们称这个配对方案在模 意义下是最优的。
例如 时,只有唯一一个最优配对方案:,其总花费为 。
我们再令一个配对方案的花费积为其所有数对的费用之积。比如说,模 意义下最优配对方案的花费积为 。
可以证明,若 ,则所有模 意义下的最优配对方案的花费积均是相同的。
试求这个花费积。
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