SICP 1.21

(define (smallest-divisor n)

  (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))

  (define (remainder a b)
	(modulo a b))

  (find-divisor n 2))

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