You have to print the sum of divisors of all the numbers from 1 to n. It’s superfast when you know the trick but you have to find it. For example if n=4, lets define d(n) the set of the divisors of n: * d(1)={1}, * d(2)={1,2}, * d(3)={1,3}, * d(4)={1,2,4}. And their sum is 1+1+2+1+3+1+2+4=15. Look carefully at the divisors to find the trick.
