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Permutations of Integers from 1 to n

Source: 2020 ISL C1

July 20, 2021
combinatoricsrecursionHi

Problem Statement

Let nn be a positive integer. Find the number of permutations a1a_1, a2a_2, …an\dots a_n of the sequence 11, 22, …\dots , nn satisfying a1≤2a2≤3a3≤⋯≤nana_1 \le 2a_2\le 3a_3 \le \dots \le na_n.
Proposed by United Kingdom
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