C1

Part of 2020 IMO Shortlist

Problems(1)

Permutations of Integers from 1 to n

Source: 2020 ISL C1

n

be a positive integer. Find the number of permutations

a_1

a_2

\dots a_n

of the sequence

1

2

\dots

n

a_1 \le 2a_2\le 3a_3 \le \dots \le na_n

.
Proposed by United Kingdom

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