1.21¶

; Use the `smallest-divisor` procedure to find the 
; smallest divisor of each of the following numbers: 
; 199, 1999, 19999.
(define (divides? a b) (= (remainder b a) 0))
(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)))))

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