Square-difference-free set (en.wikipedia.org/wiki/Square-d), now a Good Article on Wikipedia.

As the name suggests, these are sets of integers no two of which differ by a square. My favorite is the losing positions in subtract-a-square (en.wikipedia.org/wiki/Subtract), where each move removes a square number of coins from a pile of coins, winning by taking the last coin.

This general class of sets and the subtract-a-square set have $$o(n)$$ elements up to $$n$$, but their maximum density remains unknown.

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