Chapter 3: Iteration and Invariants

Perfect Numbers

A number is perfect if the sum of its divisors is twice the number.

Example: the divisors of 6 are:

	     1 + 2 + 3 + 6 = 12

Example: the divisors of 28 are:

	     1 + 2 + 4 + 7 + 14 + 28 = 56

Q: Are there any odd perfect numbers?

A procedure to test for perfect numbers:

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