SICP 問題 3.71

問題文通りに．
ラマヌジャン数のストリームを作る．

(define (sum-cube x)
  (let ((a (car x))
        (b (cadr x)))
    (+ (* a a a) (* b b b))))

(define (ramanujan stream)
  (let ((s1 (stream-car stream))
        (s2 (stream-car (stream-cdr stream))))
    (let ((weight1 (sum-cube s1))
          (weight2 (sum-cube s2)))
      (cond ((= weight1 weight2)
             (cons-stream weight1
                          (ramanujan (stream-cdr stream))))
            (else
             (ramanujan (stream-cdr stream)))))))

(define ramanujan-number
  (ramanujan (weighted-pairs integers integers sum-cube)))

gosh> (stream-head ramanujan-number 6)
1729
4104
13832
20683
32832
39312
done