Problems

Apparently, David Hilbert has -just before his 23 problems lecture in 1900- said something that people translate as

“… a mathematical problem should be difficult in order to entice us, yet not completely inaccessible, lest it mock at our efforts. …”

You won’t find those problems here, instead, some that I find interesting. A question mark (❓) denotes the problems that I am yet to solve!