The "3n + 1" problem. If a given positive integer, n, is even, it is divided by two. If n is odd, it is multiplied by three, and added to one. The Collatz Conjecture states that all positive integers n will enter an infinite loop of 4, 2, 1, 4, 2, 1, 4, 2, 1, etc. This project calculates the number of steps for an integer n to "hit one" (I have lazily left the project unable to break the loop - you will have to check when n = 1 manually). It also lists all the integers it took to get to one.
