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28 (twenty-eight) is the natural number following 27 and preceding 29. ==In mathematics== It is a composite number, its proper divisors being 1, 2, 4, 7, and 14. Twenty-eight is the second perfect number. As a perfect number, it is related to the Mersenne prime 7, since 22(23 - 1) = 28. The next perfect number is 496, the previous being 6. Twenty-eight is the third Granville number. The next Granville number is 96, the previous being 24. Twenty-eight is the sum of the totient function for the first nine integers. Twenty-eight is not the aliquot sum of any number other than itself and is therefore not a component in an aliquot sequence. Since the greatest prime factor of 282 + 1 = 785 is 157, which is more than 28 twice, 28 is a Størmer number. Twenty-eight is a harmonic divisor number, a happy number, a triangular number, a hexagonal number, 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 base 10 digits: 2, 8, 10, 18, 28... Twenty-eight is the third positive integer with a prime factorization of the form where ''q'' is an odd prime. Twenty-eight is the ninth and last number in early Indian magic square of order 3. There are twenty-eight convex uniform honeycombs. Twenty-eight is the only positive integer that has a unique Kayles nim-value. Twenty-eight is the only known number which can be expressed as a sum of the first non negative integers (1 + 2 + 3 + 4 + 5 + 6 + 7), a sum of the first primes (2 + 3 + 5 + 7 + 11) and a sum of the first non primes (1 + 4 + 6 + 8 + 9) and there is probably no other number with this property.〔http://mathoverflow.net/questions/212985〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「28 (number)」の詳細全文を読む スポンサード リンク
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