1-21.scm

#lang sicp

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

(define (square a) (* a a))

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