返回题库

HMMT 二月 2001 · 冲刺赛 · 第 32 题

HMMT February 2001 — Guts Round — Problem 32

专题
Contest Math / 竞赛数学
难度
L3
来源
HMMT

题目详情

英文原题

  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

英文解析

  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 .