問題1.21

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

(defun find-divisor (n test-divisor)
  (cond ((> (expt test-divisor 2) n) n)
        ((dividesp test-divisor n) test-divisor)
        (t (find-divisor n (1+ test-divisor)))))

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

(smallest-divisor 199)
;; => 199
(smallest-divisor 1999)
;; => 1999
(smallest-divisor 19999)
;; => 7