80
domain: N
Properties
Digital Properties
- Digit Count
- 2
- Digit Sum
- 8
- Digital Root
- 8
- Palindromic Number
- no
- Repdigit
- no
- Automorphic
- no
- Kaprekar Number
- no
- Multiplicative Persistence
- 1
Divisibility
- Divisor Count
- 10
- Divisor Sum
- 186
- Proper Divisor Sum (Aliquot Sum)
- 106
- Abundant Number
- yes
- Perfect Number
- no
- Deficient Number
- no
- Highly Composite
- no
- Weird Number
- no
- Untouchable Number
- no
- Primitive Abundant
- no
Derived Values
- Euler's Totient
- 32
- Möbius Function
- 0
- Radical
- 10
- Omega Function (Ω)
- 5
- 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
- no
- Perfect Square
- no
- Perfect Cube
- no
- Pentagonal Number
- no
- Hexagonal Number
- no
- Lucas Number
- no
- Tetrahedral Number
- no
- Pell Number
- no
- Tribonacci Number
- no
- Pronic Number
- no
Recreational
- Happy Number
- no
- Harshad Number
- yes
- Narcissistic Number
- no
- Collatz Steps
- 9
- 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
- yes
- Carmichael Number
- no
Names
- German
- achtzig· ordinal: achtzigste
- English
- eighty· ordinal: eightieth
- Spanish
- ochenta· ordinal: 80º
- French
- quatre-vingts· ordinal: quatre-vingtsième
- Italian
- ottanta· ordinal: 80º
- Latin
- octoginta· ordinal: 80.
- Portuguese
- oitenta· ordinal: 80º
Appears in sequences
- Erroneous version of A032522.at n=11A000017
- Numbers that are not squares (or, the nonsquares).at n=71A000037
- 1-digit numbers arranged in alphabetical order, then the 2-digit numbers arranged in alphabetical order, then the 3-digit numbers, etc.at n=11A000052
- Numbers k such that (2k)^4 + 1 is prime.at n=23A000059
- Generalized tangent numbers d(n,1).at n=30A000061
- A Beatty sequence: a(n) = floor(n/(e-2)).at n=57A000062
- Numbers k such that k^4 + 1 is prime.at n=14A000068
- Expansion of e.g.f. exp((-x^3)/3)/(1-x).at n=5A000090
- Number of transformation groups of order n.at n=56A000113
- Denumerants: Expansion of 1/((1-x)*(1-x^2)*(1-x^5)).at n=36A000115
- A nonlinear binomial sum.at n=7A000126
- Number of ways of writing n as a sum of 5 squares.at n=3A000132
- Ménage numbers: a(0) = 1, a(1) = -1, and for n >= 2, a(n) = number of permutations s of [0, ..., n-1] such that s(i) != i and s(i) != i+1 (mod n) for all i.at n=6A000179
- Lower Wythoff sequence (a Beatty sequence): a(n) = floor(n*phi), where phi = (1+sqrt(5))/2 = A001622.at n=49A000201
- a(n) = sigma(n), the sum of the divisors of n. Also called sigma_1(n).at n=56A000203
- A Beatty sequence: floor(n*(e-1)).at n=46A000210
- Maximum m such that there are no two adjacent elements belonging to the same n-th power residue class modulo some prime p in the sequence 1,2,...,m (equivalently, there is no n-th power residue modulo p in the sequence 1/2,2/3,...,(m-1)/m).at n=4A000236
- Number of 3-edge-connected rooted cubic maps with 2n nodes and a distinguished Hamiltonian cycle.at n=4A000264
- Two decks each have n kinds of cards, 2 of each kind. The first deck is laid out in order. The second deck is shuffled and laid out next to the first. A match occurs if a card from the second deck is next to a card of the same kind from the first deck. a(n) is the number of ways of achieving no matches.at n=3A000316
- Topswops (1): start by shuffling n cards labeled 1..n. If top card is m, reverse order of top m cards, then repeat. a(n) is the maximal number of steps before top card is 1.at n=12A000375