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

   > (perfect? 1)  =>  #f
   > (perfect? 2)  =>  #f
   > (perfect? 3)  =>  #f
   > (perfect? 4)  =>  #f
   > (perfect? 5)  =>  #f
   > (perfect? 6)  =>  #t
   > (perfect? 7)  =>  #f
   > (perfect? 8)  =>  #f
   > (perfect? 9)  =>  #f
   > (perfect? 10)  =>  #f
   > (perfect? 11)  =>  #f
   > (perfect? 12)  =>  #f
   .
   .