Euler Project 12

The sequence of triangle numbers is generated by adding the natural numbers. So the 7^th triangle number would be 1 + 2 + 3 + 4 + 5 + 6 + 7 = 28. The first ten terms would be:

1, 3, 6, 10, 15, 21, 28, 36, 45, 55, ...

Let us list the factors of the first seven triangle numbers:

1: 1
3: 1,3
6: 1,2,3,6
10: 1,2,5,10
15: 1,3,5,15
21: 1,3,7,21
28: 1,2,4,7,14,28

We can see that 28 is the first triangle number to have over five divisors.What is the value of the first triangle number to have over five hundred divisors?

My Solution

(defun factorlist(num)

(loop for i from 1 to (floor (sqrt num))

when(zerop (rem num i))

sum 2))

(defun mysum(num)

(/ (* num (+ num 1)) 2))

(defun Euler23(iter)

(loop

(if (< 500 (factorlist (mysum iter)))

(return iter)

(progn (incf iter)))))