(defun primes (n)
           (flet ((divisible-p (a b)
                    (= 1 (denominator (/ a b)))))
             (cond ((< n 2) ())
                   ((= n 2) (list 2))
                   (T (let ((primes (primes (1- n))))
                        (if (find n primes :test #'divisible-p)
                            primes
                            (list* n primes)))))))

(defun prime-factors (n)
           (let ((primes (primes (ceiling (sqrt n)))))
             (loop for prime in primes
                   for next = (/ n prime)
                   when (= 1 (denominator next))
                   collect prime
                   when (= 1 (denominator next))
                   do (setf n next))))