divisors.lisp

(defun divisors (n &aux (divisors (list 1)))
  (etypecase n
    ((integer * 1) nil)
    ((integer 2 *)
     (loop with q and r
           for i from 2 to (isqrt n)
           do (setf (values q r) (truncate n i))
           when (zerop r) do (push q divisors) and do (push i divisors))
     (if (and (second divisors)
              (= (first divisors) (second divisors)))
         (rest divisors)
       divisors))))