PUMaC 2007 · 组合（A 组） · 第 9 题
PUMaC 2007 — Combinatorics (Division A) — Problem 9
题目详情
- For how many permutations ( a , a , . . . , a ) of the integers from 1 to 2007 is there exactly one i 1 2 2007 c f between 1 and 2006 such that a > a ? Express your answer as a ∗ b + d ∗ e for integers a , b , c , i i +1 d , e , and f with a 6 | b and d 6 | e .
解析
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Original Explanation
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