sicp 1.21

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

(define (find-divisor n test-divisor)
  (cond ((> (square test-divisor) n) n)
        ((= (remainder n test-divisor) 0) test-divisor)
        (else (find-divisor n (+ test-divisor 1)))))

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