Number analysis
66
66 is a composite even number that factors as 2 × 3 × 11.
66 is a composite even number that factors as 2 × 3 × 11.
The number itself
- Value
- 66
- Digits
- 2
- Digit sum
- 12
- Reversed
- 66
- Palindrome
- Yes
6 + 6
Digit sum repeated until one digit remains; equals n mod 9 (with 9 for multiples of 9).
Reads the same in both directions.
Parity and primality
- Parity
- Even
- Prime
- No
- Prime factorisation
- 2 × 3 × 11
- Squarefree
- Yes
- Möbius μ(n)
- -1
- Nearest primes
- 61 ← → 67
Divisible by 2.
Composite — it factors as 2 × 3 × 11.
3 distinct prime factors, 3 with multiplicity.
No prime divides it twice.
(−1) to the power of 3 distinct prime factors.
A gap of 6 between them.
Divisors
- Aliquot sum
- 78
- Classification
- Abundant
1, 2, 3, 6, 11, 22, 33, 66
The proper divisors — everything except n itself.
Its proper divisors overshoot by 12.
How many integers from 1 to n share no factor with n.
Sums and additive structure
- Sum of two squares
- No
- Sum of three squares
- Yes
- Sum of four squares
- Yes
- Goldbach pair
- 5 + 61
A prime congruent to 3 mod 4 divides it an odd number of times, which rules it out.
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
- Triangular number
- Yes — the 11th
- Hexagonal number
- Yes — the 6th
Digit curiosities
- Happy number
- No
- Harshad number
- No
- Collatz trajectory
- 27 steps to 1
Summing the squares of its digits falls into the 4 → 16 → 37 → … → 4 cycle.
Not divisible by its digit sum 12.
Peaks at 100 along the way.
Written other ways
- Binary
- 1000010
- Octal
- 102
- Hexadecimal
- 42
- Base 36
- 1U
- Roman numerals
- LXVI
- In words
- sixty-six
- Scientific notation
- 6.6 × 10^1
Is 66 a prime number?
Composite — it factors as 2 × 3 × 11.
Nearby numbers
Other numeral systems
Beyond the bases and Roman numerals above, these notations write 66 their own way.
- Greek (alphabetic)
- ΞΣΤʹ
- Babylonian cuneiform
- 𒐕𒐚
- Tamil
- ௬௰௬
References
Further reading: Wikipedia · 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.