Number analysis
18
18 is a composite even number that factors as 2 × 3^2.
18 is a composite even number that factors as 2 × 3^2.
The number itself
- Value
- 18
- Digits
- 2
- Digit sum
- 9
- Reversed
- 81
- Palindrome
- No
1 + 8
Digit sum repeated until one digit remains; equals n mod 9 (with 9 for multiples of 9).
Reverses to 81.
Parity and primality
- Parity
- Even
- Prime
- No
- Prime factorisation
- 2 × 3^2
- Squarefree
- No
- Möbius μ(n)
- 0
- Nearest primes
- 17 ← → 19
Divisible by 2.
Composite — it factors as 2 × 3^2.
2 distinct prime factors, 3 with multiplicity.
At least one prime appears squared.
Zero, because a squared prime divides n.
A gap of 2 between them.
Divisors
- Aliquot sum
- 21
- Classification
- Abundant
1, 2, 3, 6, 9, 18
The proper divisors — everything except n itself.
Its proper divisors overshoot by 3.
How many integers from 1 to n share no factor with n.
Sums and additive structure
- Sum of two squares
- Yes
- Sum of three squares
- Yes
- Sum of four squares
- Yes
- Goldbach pair
- 5 + 13
3² + 3² = 18
Legendre's three-square theorem allows it.
Lagrange's four-square theorem: every non-negative integer is, without exception.
The smallest pair of primes summing to n. Goldbach’s conjecture says one always exists; it remains unproven.
Shapes and sequences
- Lucas number
- Yes — index 6
Digit curiosities
- Happy number
- No
- Harshad number
- Yes
- Collatz trajectory
- 20 steps to 1
Summing the squares of its digits falls into the 4 → 16 → 37 → … → 4 cycle.
Divisible by its digit sum 9 — 18 ÷ 9 = 2.
Peaks at 52 along the way.
Written other ways
- Binary
- 10010
- Octal
- 22
- Hexadecimal
- 12
- Base 36
- I
- Roman numerals
- XVIII
- In words
- eighteen
- Scientific notation
- 1.8 × 10^1
Is 18 a prime number?
Composite — it factors as 2 × 3^2.
Nearby numbers
Other numeral systems
Beyond the bases and Roman numerals above, these notations write 18 their own way.
- Greek (alphabetic)
- ΙΗʹ
- Babylonian cuneiform
- 𒌋𒐜
- Tamil
- ௰௮
References
Further reading: Wikipedia · MathWorld · Wikidata
Notations, naming and the links above come from Wikidata, whose structured data is dedicated to the public domain under CC0-1.0. Retrieved 2026-10-07.