SICP 1.2.6 素数检测

O(n√)的复杂度

(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 (prime? n)
(= n (smallest-divisor n)))

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