Number reference
66 explained.
66: sixty-six, composite, triangular number; factors, prime factorization, roots, binary, hexadecimal, and digit properties.
Identity and representations
- Written form
- sixty-six
- Classification
- composite
- Factors
- 1, 2, 3, 6, 11, 22, 33, 66
- Prime factorization
- 2 × 3 × 11
- Square root
- 8.124038
- Cube root
- 4.04124
- Binary
- 1000010
- Octal
- 102
- Hexadecimal
- 42
- Roman numeral
- LXVI
- Digit sum
- 12
- Digital root
- 3
What makes 66 notable
- Triangular number. 66 is the 11th triangular number: it is the sum 1 + 2 + … + 11, the count of items in a triangle with 11 rows.
- Repdigit. 66 is written with a single repeated digit, so it is divisible by the repunit of the same length.
66 in everyday use
Route 66 ran from Chicago to Santa Monica from 1926 until it was formally removed from the highway system in 1985, and it survives in song, television, and roadside nostalgia. Sixty-six is the atomic number of dysprosium, a rare earth in hard-drive magnets. The Protestant Bible contains 66 books, 39 in the Old Testament and 27 in the New. Hockey great Mario Lemieux wore 66, deliberately inverting Wayne Gretzky's 99.
Sixty-six is the eleventh triangular number, 1 + 2 + … + 11, and a repdigit, 6 × 11. It is a sphenic number, the product of three different primes: 2 × 3 × 11. It has eight divisors, and its proper divisors sum to 78, so it is abundant. In binary it is 1000010, and it is also the number of ways to choose two items from twelve.
Nearby numbers
How to read this page
The factor list shows every way 66 splits into equal whole-number groups — 8 divisors in total. The prime factorisation is the compact form of that same information, and the base conversions restate 66 for contexts that do not use decimal.
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