28 (number)

← 27 28 29 →
← 20 21 22 23 24 25 26 27 28 29 → List of numbers — Integers ← 0 10 20 30 40 50 60 70 80 90 →
Cardinal	twenty-eight
Ordinal	28th (twenty-eighth)
Factorization	22× 7
Divisors	1, 2, 4, 7, 14, 28
Roman numeral	XXVIII
Binary	111002
Ternary	10013
Quaternary	1304
Quinary	1035
Senary	446
Octal	348
Duodecimal	2412
Hexadecimal	1C16
Vigesimal	1820
Base 36	S36

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

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 2^{(3 − 1)}(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.

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