(define divides?
  (lambda (d n)
    (= (remainder n d) 0)))

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

(define perfect?
  (lambda (n)
    (= (sum-of-divisors n) (* 2 n))))





(define sum-of-divisors
  (lambda (n)
    (define sum-from-plus ; sum of all divisors of n which are
      (lambda (low addend) ; >= low and <= n/low, plus addend
        (cond
          ((> (square low) n) addend )
          ((= (square low) n) (+ addend low))
          (else (sum-from-plus (+ low 1)
                               (if (divides? low n)
                                   (+ addend low (/ n low))
                                   addend))))))
    (sum-from-plus 1 0)))









(define abundant/deficient
  (lambda (n)
    (define ratio-from
      (lambda (k abun def); the ratio (abun + abundant #'s between k and n
        (if                ;divided by (def + deficient #'s between k and n)
         (> k n)
         (if (not (= 0 def)) (/ abun def) -17)
         (ratio-from (+ k 1)
                     (if (> (sum-of-divisors k) (* 2 k))
                         (+ abun 1)
                         abun)
                     (if (< (sum-of-divisors k)(* 2 k))
                         (+ def 1)
                         def)))))
    (ratio-from 1 0 0)))