Exercise 1.21

文責:@iriya_ufo

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

(define (find-divisor n test-divisor)
  (cond ((> (square test-divisor) n) n)
        ((divides? test-divisor n) test-divisor)
        (else (find-divisor n (+ test-divisor 1)))))

(define (square x) (* x x))

(define (divides? a b)
  (= (remainder b a) 0))

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

199, 1999, 19999 の最小除数はそれぞれ 199, 1999, 7 である. よって 19999 は素数でないことが分かる.