You may find the Collatz conjecture interesting: https://en.wikipedia.org/wiki/Collatz_conjecture https://en.wikipedia.org/wiki/Collatz_conjecture It's an easy

You may find the Collatz conjecture interesting: https://en.wikipedia.org/wiki/Collatz_conjecture https://en.wikipedia.org/wiki/Collatz_conjecture It's an easy to understand loop, which can be written as a recursive function of one argument. Even though the possible arguments are just the natural numbers, nobody knows how to identify a suitable ordering relation which decreases with each call.