28 (twenty-eight) is the natural number following 27 and preceding 29.

In mathematics

Twenty-eight is a composite number and the second perfect number as it is the sum of its proper divisors:. As a perfect number, it is related to the Mersenne prime 7, since. 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. The next perfect number is 496. Twenty-eight is the sum of the totient function for the first nine integers. Since the greatest prime factor of is 157, which is more than 28 twice, 28 is a Størmer number. Twenty-eight is a harmonic divisor number, a happy number, the 7th triangular number, a hexagonal number, a Leyland number of the second kind (2^6-6^2), and a centered nonagonal number. It appears in the Padovan sequence, preceded by the terms 12, 16, 21 (it is the sum of the first two of these). It is also a Keith number, because it recurs in a Fibonacci-like sequence started from its decimal digits: 2, 8, 10, 18, 28... There are 28 convex uniform honeycombs. Twenty-eight is the only positive integer that has a unique Kayles nim-value. Twenty-eight is the only known number that can be expressed as a sum of the first positive integers, a sum of the first primes , and a sum of the first nonprimes , and it is unlikely that any other number has this property. There are twenty-eight oriented diffeomorphism classes of manifolds homeomorphic to the 7-sphere. There are 28 non-equivalent ways of expressing 1000 as the sum of two prime numbers. Twenty-eight is the smallest number that can be expressed as the sum of four nonzero squares in (at least) three ways:, or (see image).

In science

In astronomy

In sports

In other fields

Twenty-eight is: