| ||||
---|---|---|---|---|
Cardinal | twenty-eight | |||
Ordinal | 28th (twenty-eighth) | |||
Factorization | 22 × 7 | |||
Divisors | 1, 2, 4, 7, 14, 28 | |||
Greek numeral | ΚΗ´ | |||
Roman numeral | XXVIII | |||
Binary | 111002 | |||
Ternary | 10013 | |||
Senary | 446 | |||
Octal | 348 | |||
Duodecimal | 2412 | |||
Hexadecimal | 1C16 |
28 (twenty-eight) is the natural number following 27 and preceding 29.
It is a composite number; a square-prime, of the form (p2,q) where q is a higher prime. It is the third of this form and of the specific form (22.q), with proper divisors being 1, 2, 4, 7, and 14.
Twenty-eight is the second perfect number - 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. [1]
Though perfect, 28 is not the aliquot sum of any other number other than itself, and so; unusually, 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. [2]
Since the greatest prime factor of is 157, which is more than 28 twice, 28 is a Størmer number. [3]
Twenty-eight is a harmonic divisor number, [4] a happy number, [5] a triangular number, [6] a hexagonal number, [7] a Leyland number of the second kind and a centered nonagonal number. [8]
It appears in the Padovan sequence, preceded by the terms 12, 16, 21 (it is the sum of the first two of these). [9]
It is also a Keith number, because it recurs in a Fibonacci-like sequence started from its decimal digits: 2, 8, 10, 18, 28... [10]
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 that can be expressed as a sum of the first nonnegative (or 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. [11]
There are twenty-eight oriented diffeomorphism classes of manifolds homeomorphic to the 7-sphere.[ citation needed ]
There are 28 elements of the cuboid: 8 vertices, 12 edges, 6 faces, 2 3-dimensional elements (interior and exterior).
There are 28 non-equivalent ways of expressing 1000 as the sum of two prime numbers [12]
The cube of 12 (1728 = 123) contains a total of twenty-eight divisors (the third-smallest number after 1344 and 960, and preceding 2112).
28 is the smallest number that can be expressed as the sum of four nonzero squares in (at least) three ways: , or (see image). [13] [14]
Twenty-eight is:
21 (twenty-one) is the natural number following 20 and preceding 22.
33 (thirty-three) is the natural number following 32 and preceding 34.
70 (seventy) is the natural number following 69 and preceding 71.
90 (ninety) is the natural number following 89 and preceding 91.
29 (twenty-nine) is the natural number following 28 and preceding 30. It is a prime number.
27 is the natural number following 26 and preceding 28.
72 (seventy-two) is the natural number following 71 and preceding 73. It is half a gross or six dozen.
84 (eighty-four) is the natural number following 83 and preceding 85.
34 (thirty-four) is the natural number following 33 and preceding 35.
31 (thirty-one) is the natural number following 30 and preceding 32. It is a prime number.
58 (fifty-eight) is the natural number following 57 and preceding 59.
92 (ninety-two) is the natural number following 91 and preceding 93.
104 is the natural number following 103 and preceding 105.
360 is the natural number following 359 and preceding 361.
127 is the natural number following 126 and preceding 128. It is also a prime number.
135 is the natural number following 134 and preceding 136.
168 is the natural number following 167 and preceding 169.
177 is the natural number following 176 and preceding 178.
1728 is the natural number following 1727 and preceding 1729. It is a dozen gross, or one great gross. It is also the number of cubic inches in a cubic foot.
888 is the natural number following 887 and preceding 889.