Prime factors are defined by a pretty rigid structure; you see a factor of 2 every two numbers; a factor of 3 every three numbers; a factor of 5 every five numbers...
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
0 1 0 2 0 1 0 3 0 1 0 2 0 1 0