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Omni-MATH / We are given 2n natural numbers

Problem

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problem

We are given 2n2n natural numbers 1,1,2,2,3,3,…,n−1,n−1,n,n.1, 1, 2, 2, 3, 3, \ldots, n - 1, n - 1, n, n. Find all nn for which these numbers can be arranged in a row such that for each k≤nk \leq n, there are exactly kk numbers between the two numbers kk.
Plain-text mathematical notation (without MathML)
We are given 2n natural numbers
1,1,2,2,3,3,…,n−1,n−1,n,n.
Find all n for which these numbers can be arranged in a row such that for each k≤n, there are exactly k numbers between the two numbers k.
Original LaTeX notation
We are given $2n$ natural numbers
\[1, 1, 2, 2, 3, 3, \ldots, n - 1, n - 1, n, n.\]
Find all $n$ for which these numbers can be arranged in a row such that for each $k \leq n$, there are exactly $k$ numbers between the two numbers $k$.

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