The Collatz conjecture is quite possibly the simplest unsolved problem in mathematics.
Also known as Conjecture of Ulam or problem of Syracuse.
Well, still its just a squence, but unsolved...
The Sequence should be like this:
Take any positive integer n.
If n is even, divide it by 2 to get n/2.
If n is odd, multiply it by 3 and add 1 to obtain 3n+1.
Try it!