SICP习题 1.23(素数查找的去偶数优化)

(define (next n)
  (if (= n 2)
      3
      (+ n 2)))

(define (find-divisor n test-divisor)
  (cond ((> (square test-divisor) n) n)
        ((divides? n test-divisor) test-divisor)
        (else (find-divisor n (next test-divisor)))))

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

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

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

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

(smallest-divisor 177)