The sum of the primes below 10 is 2 + 3 + 5 + 7 = 17. Find the sum of all the primes below two million.

This solution tests if a number is prime by testing if the number's smallest divisor is equal to itself. It then loops through all the odd numbers by two and sums the primes.

;;; Functions for testing if a number is prime
(defun square (x) (* x x))

(defun dividesp (a b)
  (zerop (mod b a)))

(defun find-divisor (n test-divisor)
  (cond ((> (square test-divisor) n) n)
        ((dividesp test-divisor n) test-divisor)
        (t (find-divisor n (+ test-divisor 1)))))

(defun smallest-divisor (n)
  (find-divisor n 2))

(defun primep (n)
  (= n (smallest-divisor n)))

;;; End of prime functions
  
(defun sum-of-primes-below (num)
  "Returns the sum of all the prime numbers below num"
  (+ 2 (loop for i from 3 to num by 2 when (primep i) sum i)))

(defun euler-10 ()
  (sum-of-primes-below 2000000))