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Oct 4, 2021 at 14:13 vote accept Kai Wang
Oct 1, 2021 at 20:22 history edited Fedor Petrov CC BY-SA 4.0
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Oct 1, 2021 at 19:47 comment added GH from MO Let $S$ be the OP's set. I think we can simply define the color of a positive integer $n$ as $\min\{i\geq 1:n+i\in S\}$. By assumption, the color of each $n$ is bounded by $T$, so we used finitely many colors. By van der Waerden's theorem, there is an arbitrary long arithmetic progression $P$ which is monochromatic. If the color of $P$ is $i$, then $P+i$ is a long arithmetic progression in $S$.
Oct 1, 2021 at 19:36 history answered Fedor Petrov CC BY-SA 4.0