(include-book "arithmetic-3/top" :dir :system)

(defun prime-factors-r (n i)
   (declare (xargs :mode :program))
   (cond ((or (zp n) (zp (- n i)) (zp i) (< i 2) (< n 2))
          (list n))
         ((= (mod n i) 0)
          (cons i (prime-factors-r (floor n i) 2)))
         (t (prime-factors-r n (1+ i)))))

(defun prime-factors (n)
   (declare (xargs :mode :program))
   (prime-factors-r n 2))