Number I find interesting is:
The Collatz Conjecture is a conjecture in mathematics that concerns a sequence defined as follows:
start with any positive integer n. Then each term is obtained from the previous term as follows:
if the previous term is even, the next term is one half of the previous term. If the previous term is odd,
the next term is 3 times the previous term plus 1.
The conjecture is that no matter what value of n, the sequence will always reach 1.