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.




 
F := unapply(  sum(3*i,i=1..floor((n-1)/3))
             + sum(5*i,i=1..floor((n-1)/5))
             - sum(15*i,i=1..floor((n-1)/15)), n);
 
F(1000);
 
F(10^20);
 