Every positive integer except for zero includes 1. This is apparent 5*1=5. Therefore, every positive integer must be reduced to 1 . This is called Collatz problem. When you have even numbers, you divide them by 2. Then if the integer is the even number, you divide it by 2. However, 10/2 is 5. In this case, you have odd numbers, so you multiply 3 to them and you plus 1 to it. You repeat over and over again, until you reach to 1. ex. 18/2=9, 9*3+1=28, 28/2=14, 14/2=7, 7*3+1=22, 22/2=11, 11*3+1=34, 34/2=17, 17*3+1=52, 52/2=26, 26/2=13, 13*3+1=40, 40/2=20, 20/2=10, 10/2=5, 5*3+1=16, 16/2=8, 8/2=4, 4/2=2, 2/2=1