I would say the collatz conjecture is up there. It's been an entry point for many people into the computational side of mathematics. Take any positive whole nu

I would say the collatz conjecture is up there. It's been an entry point for many people into the computational side of mathematics. Take any positive whole number, n. If it is even, divide it by 2. (n = 2n) If it odd, multiply it by 3 and add one. (n = 3n + 1) Repeat. If n = 1, stop. Does this process always lead to 1? It seems to, but who knows. https://en.wikipedia.org/wiki/Collatz_conjecture https://en.wikipedia.org/wiki/Collatz_conjecture