Number analysis
91
91 is a composite odd number that factors as 7 × 13.
91 is a composite odd number that factors as 7 × 13.
The number itself
- Value
- 91
- Digits
- 2
- Digit sum
- 10
- Reversed
- 19
- Palindrome
- No
9 + 1
Digit sum repeated until one digit remains; equals n mod 9 (with 9 for multiples of 9).
Reverses to 19.
Parity and primality
- Parity
- Odd
- Prime
- No
- Prime factorisation
- 7 × 13
- Squarefree
- Yes
- Semiprime
- Yes
- Möbius μ(n)
- 1
- Nearest primes
- 89 ← → 97
Leaves remainder 1 on division by 2.
Composite — it factors as 7 × 13.
2 distinct prime factors, 2 with multiplicity.
No prime divides it twice.
A product of exactly two primes, counted with multiplicity.
(−1) to the power of 2 distinct prime factors.
A gap of 8 between them.
Divisors
- Aliquot sum
- 21
- Classification
- Deficient
1, 7, 13, 91
The proper divisors — everything except n itself.
Its proper divisors fall short by 70.
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
- Sum of two cubes
- 3³ + 4³
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
- Triangular number
- Yes — the 13th
- Hexagonal number
- Yes — the 7th
Digit curiosities
- Happy number
- Yes
- Harshad number
- No
- Collatz trajectory
- 92 steps to 1
Repeatedly summing the squares of its digits reaches 1.
Not divisible by its digit sum 10.
Peaks at 9,232 along the way.
Written other ways
- Binary
- 1011011
- Octal
- 133
- Hexadecimal
- 5B
- Base 36
- 2J
- Roman numerals
- XCI
- In words
- ninety-one
- Scientific notation
- 9.1 × 10^1
Is 91 a prime number?
Composite — it factors as 7 × 13.
Nearby numbers
Other numeral systems
Beyond the bases and Roman numerals above, these notations write 91 their own way.
- Greek (alphabetic)
- ϞΑʹ
- Babylonian cuneiform
- 𒐕𒌍𒐕
- Tamil
- ௯௰௧
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.