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HMMT 二月 2001 · 冲刺赛 · 第 32 题

HMMT February 2001 — Guts Round — Problem 32

专题
Discrete Math / 离散数学
难度
L3
来源
HMMT

题目详情

  1. [10] Count the number of permutations a a . . . a of 1234567 with longest decreasing 1 2 7 subsequence of length at most two (i.e. there does not exist i < j < k such that a > a > a ). i j k
解析
  1. [10] Count the number of permutations a a . . . a of 1234567 with longest decreasing 1 2 7 subsequence of length at most two (i.e. there does not exist i < j < k such that a > a > a ). i j k Solution: C (7) = 429 .