Let \(\mathcal{A}_n\) be the collection of all sequences \( \mathbf{a}= (a_1,\dots, a_n) \) with \(a_i \in [i]\) for all \(i\in [n]=\{1,2,\dots, n\}\). A nondecreasing \(k\)-subsequence of \(\mathbf{a}\) is a subsequence \( (a_{i_1}, a_{i_2},\dots, a_{i_k}) \) such that \(i_1< i_2< \dots < i_k\) and \(a_{i_1}\leq a_{i_2}\leq \dots \leq a_{i_k}\). For given \(k\), determine the smallest \(n\) such that any sequence \(\mathbf{a}\in \mathcal{A}_n\) has a nondecreasing \(k\)-subsequence.

The best solution was submitted by 고성훈 (수리과학과 2018학번, +4). Congratulations!

Here is his solution of problem 2021-06.

Another solution was submitted by 강한필 (전산학부 2016학번, +3).