I am very interested by problem in mathematics which are difficult (go at least 10 years without a resolution, say) but which have a solution that is short and elementary.

In <a href="https://www.youtube.com/watch?v=knyQyV4aXrA">this video</a> Launay gave an example, namely <a href="https://arxiv.org/abs/2403.13855">A non-terminating game of Beggar-My-Neighbor</a> (<a href="https://www.jsbeasley.co.uk/">incorrectly claimed</a> in <i>Winning Ways</i> to have been solved a long time ago).

Launay calls these problems "Pokémon problems": difficult to catch but once you catch them, they're in your pocket.

**Question**: What are other problems like this?