A perfect number is a positive integer that is equal to the sum of its positive factors, excluding the integer itself.  For example, $28$ has factors $1, 2, 4, 7, 14$ and itself.  Since $1 + 2 + 4 + 7 + 14 = 28$, we say that $28$ is a perfect number.  Find the smallest perfect number. 
Overseas Competition->Knowledge Point->Number Theory Modules->Factors and Multiples->Theorem of the Number of Factors of a Number->Sum of Factors