Number analysis
99
99 is a composite odd number that factors as 3^2 × 11.
99 is a composite odd number that factors as 3^2 × 11.
The number itself
- Value
- 99
- Digits
- 2
- Digit sum
- 18
- Reversed
- 99
- Palindrome
- Yes
9 + 9
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
- Odd
- Prime
- No
- Prime factorisation
- 3^2 × 11
- Squarefree
- No
- Möbius μ(n)
- 0
- Nearest primes
- 97 ← → 101
Leaves remainder 1 on division by 2.
Composite — it factors as 3^2 × 11.
2 distinct prime factors, 3 with multiplicity.
At least one prime appears squared.
Zero, because a squared prime divides n.
A gap of 4 between them.
Divisors
- Aliquot sum
- 57
- Classification
- Deficient
1, 3, 9, 11, 33, 99
The proper divisors — everything except n itself.
Its proper divisors fall short by 42.
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
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.
Shapes and sequences
- Figurate shapes
- None
Not triangular, square, pentagonal, hexagonal, cubic, Fibonacci, Lucas, Catalan, factorial or a power of two.
Digit curiosities
- Happy number
- No
- Harshad number
- No
- Kaprekar number
- Yes
- Collatz trajectory
- 25 steps to 1
Summing the squares of its digits falls into the 4 → 16 → 37 → … → 4 cycle.
Not divisible by its digit sum 18.
Peaks at 448 along the way.
Written other ways
- Binary
- 1100011
- Octal
- 143
- Hexadecimal
- 63
- Base 36
- 2R
- Roman numerals
- XCIX
- In words
- ninety-nine
- Scientific notation
- 9.9 × 10^1
Is 99 a prime number?
Composite — it factors as 3^2 × 11.
Nearby numbers
Other numeral systems
Beyond the bases and Roman numerals above, these notations write 99 their own way.
- Greek (alphabetic)
- ϞΘʹ
- Babylonian cuneiform
- 𒐕𒌍𒐝
- Tamil
- ௯௰௯
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.