28
domain: N
Properties
Digital Properties
- Digit Count
- 2
- Digit Sum
- 10
- Digital Root
- 1
- Palindromic Number
- no
- Repdigit
- no
- Automorphic
- no
- Kaprekar Number
- no
- Multiplicative Persistence
- 2
Divisibility
- Divisor Count
- 6
- Divisor Sum
- 56
- Proper Divisor Sum (Aliquot Sum)
- 28
- Abundant Number
- no
- Perfect Number
- yes
- Deficient Number
- no
- Highly Composite
- no
- Weird Number
- no
- Untouchable Number
- no
- Primitive Abundant
- no
Derived Values
- Euler's Totient
- 12
- Möbius Function
- 0
- Radical
- 14
- Omega Function (Ω)
- 3
- Little Omega Function (ω)
- 2
Special
- Factorial
- no
- Catalan Number
- no
- Bell Number
- no
- Motzkin Number
- no
- Primorial
- no
Figurate Numbers
- Fibonacci Number
- no
- Triangular Number
- yes
- Perfect Square
- no
- Perfect Cube
- no
- Pentagonal Number
- no
- Hexagonal Number
- yes
- Lucas Number
- no
- Tetrahedral Number
- no
- Pell Number
- no
- Tribonacci Number
- no
- Pronic Number
- no
Recreational
- Happy Number
- yes
- Harshad Number
- no
- Narcissistic Number
- no
- Collatz Steps
- 18
- Smith Number
- no
Classification
- Natural
- yes
- Even
- yes
- Odd
- no
Primality
- Prime
- no
- Composite Number
- yes
- Semiprime
- no
- Squarefree Number
- no
- Prime Power
- no
- Prime Factorization
- no
- Twin Prime
- no
- Mersenne Prime
- no
- Sophie Germain Prime
- no
- Safe Prime
- no
- Powerful Number
- no
- Achilles Number
- no
- Perfect Power
- no
- Smooth Number
- no
- Carmichael Number
- no
Names
- German
- achtundzwanzig· ordinal: achtundzwanzigste
- English
- twenty-eight· ordinal: twenty-eighth
- Spanish
- veintiocho· ordinal: 28º
- French
- vingt-huit· ordinal: vingt-huitième
- Italian
- ventotto· ordinal: 28º
- Latin
- viginti octo· ordinal: 28.
- Portuguese
- vinte e oito· ordinal: 28º
Appears in sequences
- Number of ways of making change for n cents using coins of 1, 2, 5, 10 cents.at n=17A000008
- Euler totient function phi(n): count numbers <= n and prime to n.at n=28A000010
- Euler totient function phi(n): count numbers <= n and prime to n.at n=57A000010
- Number of primitive permutation groups of degree n.at n=24A000019
- Mosaic numbers or multiplicative projection of n: if n = Product (p_j^k_j) then a(n) = Product (p_j * k_j).at n=27A000026
- The positive integers. Also called the natural numbers, the whole numbers or the counting numbers, but these terms are ambiguous.at n=27A000027
- Numbers that are not squares (or, the nonsquares).at n=22A000037
- Number of n-bead necklaces with beads of 2 colors and primitive period n, when turning over is not allowed but the two colors can be interchanged.at n=9A000048
- Local stops on New York City 1 Train (Broadway-7 Avenue Local) subway.at n=3A000053
- Numbers k such that (2k)^4 + 1 is prime.at n=11A000059
- Numbers k such that k^4 + 1 is prime.at n=7A000068
- Odious numbers: numbers with an odd number of 1's in their binary expansion.at n=14A000069
- Number of partitions of n if there are two kinds of 1's and two kinds of 2's.at n=5A000097
- Number of partitions into non-integral powers.at n=4A000148
- a(n) = sigma(n), the sum of the divisors of n. Also called sigma_1(n).at n=11A000203
- Number of even sequences with period 2n.at n=5A000208
- One-half of number of permutations of [n] with exactly one run of adjacent symbols differing by 1.at n=4A000239
- a(n) = 3*(2*n)!/((n+2)!*(n-1)!).at n=4A000245
- From a fractal set of positive Lebesgue measure, a self-replicating tiling with holes, the 4-reptile following the 2-reptile of Paul Levy.at n=21A000361
- Number of connected graphs with one cycle of length 4.at n=4A000368