0028

Twenty-eight is a composite number and the second perfect number as it is the sum of its proper divisors: 1 + 2 + 4 + 7 + 14 = 28 {\displaystyle 1+2+4+7+14=28}. As a perfect number, it is related to the Mersenne prime 7, since 2 3 − 1 × ( 2 3 − 1 ) = 28 {\displaystyle 2^{3-1}\times (2^{3}-1)=28}. The next perfect number is 496, the previous being 6. Though perfect, 28 is not the aliquot sum of any other number other than itself; thus, it is not part of a multi-number aliquot sequence. Twenty-eight is the sum of the totient function for the first nine integers. Since the greatest prime factor of 28 2 + 1 = 785 {\displaystyle 28^{2}+1=785} is 157, which is more than 28 twice, 28 is a Størmer number.