18
domain: N
Properties
Digital Properties
- Digit Count
- 2
- Digit Sum
- 9
- Digital Root
- 9
- Palindromic Number
- no
- Repdigit
- no
- Automorphic
- no
- Kaprekar Number
- no
- Multiplicative Persistence
- 1
Divisibility
- Divisor Count
- 6
- Divisor Sum
- 39
- Proper Divisor Sum (Aliquot Sum)
- 21
- Abundant Number
- yes
- Perfect Number
- no
- Deficient Number
- no
- Highly Composite
- no
- Weird Number
- no
- Untouchable Number
- no
- Primitive Abundant
- yes
Derived Values
- Euler's Totient
- 6
- Möbius Function
- 0
- Radical
- 6
- 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
- no
- Perfect Square
- no
- Perfect Cube
- no
- Pentagonal Number
- no
- Hexagonal Number
- no
- Lucas Number
- yes
- Tetrahedral Number
- no
- Pell Number
- no
- Tribonacci Number
- no
- Pronic Number
- no
Recreational
- Happy Number
- no
- Harshad Number
- yes
- Narcissistic Number
- no
- Collatz Steps
- 20
- 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
- achtzehn· ordinal: achtzehnte
- English
- eighteen· ordinal: eighteenth
- Spanish
- dieciocho· ordinal: decimoctavo
- French
- dix-huit· ordinal: dix-huitième
- Italian
- diciotto· ordinal: 18º
- Latin
- duodeviginti· ordinal: 18.
- Portuguese
- dezoito· ordinal: 18º
Appears in sequences
- Integer part of square root of n-th prime.at n=66A000006
- Integer part of square root of n-th prime.at n=67A000006
- Integer part of square root of n-th prime.at n=68A000006
- Integer part of square root of n-th prime.at n=69A000006
- Integer part of square root of n-th prime.at n=70A000006
- Expansion of Product_{m >= 1} (1 + x^m); number of partitions of n into distinct parts; number of partitions of n into odd parts.at n=13A000009
- Euler totient function phi(n): count numbers <= n and prime to n.at n=18A000010
- Euler totient function phi(n): count numbers <= n and prime to n.at n=26A000010
- Euler totient function phi(n): count numbers <= n and prime to n.at n=37A000010
- Euler totient function phi(n): count numbers <= n and prime to n.at n=53A000010
- Number of n-bead necklaces (turning over is allowed) where complements are equivalent.at n=8A000011
- Number of primitive polynomials of degree n over GF(2) (version 2).at n=6A000020
- Mosaic numbers or multiplicative projection of n: if n = Product (p_j^k_j) then a(n) = Product (p_j * k_j).at n=23A000026
- Mosaic numbers or multiplicative projection of n: if n = Product (p_j^k_j) then a(n) = Product (p_j * k_j).at n=53A000026
- The positive integers. Also called the natural numbers, the whole numbers or the counting numbers, but these terms are ambiguous.at n=17A000027
- Number of necklaces with n beads of 2 colors, allowing turning over (these are also called bracelets).at n=7A000029
- Let A(n) = #{(i,j): i^2 + j^2 <= n}, V(n) = Pi*n, P(n) = A(n) - V(n); A000099 gives values of n where |P(n)| sets a new record; sequence gives closest integer to P(A000099(n)).at n=16A000036
- Numbers that are not squares (or, the nonsquares).at n=13A000037
- 1-digit numbers arranged in alphabetical order, then the 2-digit numbers arranged in alphabetical order, then the 3-digit numbers, etc.at n=10A000052
- Local stops on New York City 1 Train (Broadway-7 Avenue Local) subway.at n=1A000053