IMO 2020 Shortlist C1

Let n be a positive integer. Find the number of permutations a1,a2,...,an of the sequence 1,2,...,n satisfying a1 ď 2a2 ...

imomathematicsolympiadshortlistcombinatorics

IMO 2020 Shortlist C1

Category: Combinatorics

Problem

Let n be a positive integer. Find the number of permutations a1,a2,...,an of the sequence 1,2,...,n satisfying a1 ď 2a2 ď 3a3 ď ... ď nan. (United Kingdom)