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Omni-MATH / Let n be a positive integer. Find the number of permutations a₁, a₂, …a_(n) of the sequence 1, 2, … , n satisfying a₁≤2a₂≤3a₃≤…≤na_(n).

Problem

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problem

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.
Plain-text mathematical notation (without MathML)
Let n be a positive integer. Find the number of permutations a₁, a₂, …a_(n) of the
sequence 1, 2, … , n satisfying
a₁≤2a₂≤3a₃≤…≤na_(n).
Original LaTeX notation
Let $n$ be a positive integer. Find the number of permutations $a_1$, $a_2$, $\dots a_n$ of the
sequence $1$, $2$, $\dots$ , $n$ satisfying
$$a_1 \le 2a_2\le 3a_3 \le \dots \le na_n$$.

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Source and history

task_id
7597d5e0-4c94-5876-a48b-dfddbc3cbde5
task_key
test--7597d5e0-4c94-5876-a48b-dfddbc3cbde5
task_revision_id
1
config / split
/ test

Official source

initial import