Hamming's problem of number generation

A famous problem, first raised by R. Hamming, is to enumerate, in ascending order with no repetitions, all positive integers with no prime factors other than 2, 3 , or 5 . One obvious way to do this is to simply test each integer in turn to see whether it has any factors other than 2, 3 , and 5 . But this is very inefficient, since, as the integers get larger, fewer and fewer of them fit the requirement. As an alternative, let us call the required stream of numbers S and notice the following facts about it.

• S begins with 1 .

• The elements of (scale-streams 2) are also elements of S .

• The same is true for (scale-stream S 3) and (scale-stream S 5) .

• These are all the elements of S .

Now all we have to do is combine elements from these sources. For this we define a procedure merge that combines two ordered streams into one ordered result stream, eliminating repetitions:

(define (merge s1 s2)
  (cond ((stream-null? s1) s2)
        ((stream-null? s2) s1)
        (else
         (let ((s1car (stream-car s1))
               (s2car (stream-car s2)))
           (cond ((< s1car s2car)
                  (cons-stream s1car (merge (stream-cdr s1) s2)))
                 ((> s1car s2car)
                  (cons-stream s2car (merge s1 (stream-cdr s2))))
                 (else
                  (cons-stream s1car
                               (merge (stream-cdr s1)
                                      (stream-cdr s2)))))))))

Then the required stream may be constructed with merge , as follows:

(define S (cons-stream 1 (merge <??> <??>)))

Fill in the missing expressions in the places marked <??> above.

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