Six is also the first perfect number which means that the sum of its factors ( 1 , 2 and 3 ) are equal to the number itself ( 6 ) . Since six equals the sum of these proper divisors , six is the smallest perfect number , Granville number , and - perfect number . As a perfect number , 6 is related to the Mersenne prime 3 , since 21 ( 22 - 1 ) = 6 .