Q: Are there any odd perfect numbers?
If there are any, then the following procedure would
eventually find it and halt:
(define find-perfect-odd
(lambda (n) ; start n at 1
(if (perfect? n)
n
(find-perfect-odd (+ n 2)))))
Since there may not be enough time left in the universe to wait,
we could ask:
(halts? find-perfect-odd) ⇒ <the magic answer>
While halts? sounds too good to be true, this is not a proof that it
doesn't exist.