I am very interesting by the problem of mathematics, which are hard to resolved (go at least 10 years without a response) and with a solution easy to understand (less than 10 lines license levels).

In this video Launay give an example : https://www.youtube.com/watch?v=knyQyV4aXrA

Launay calls these problems pokemon problems, difficult to catch but once you catch them, you're in your pocket.

>I'm asking for a big list of pokemon problems.