Task

The objective is to write a function that finds the sum of all positive multiples of 3 or 5 below n.

Show output for n = 1000.

This is is the same as Project Euler problem 1.

Extra credit: do this efficiently for n = 1e20 or higher.




ct(n,k)=n=n--\k;k*n*(n+1)/2;
a(n)=ct(n,3)+ct(n,5)-ct(n,15);
a(1000)
a(1e20)