91
91 is the atomic number of protactinium, factoring as 7 × 13 despite ending in the digit 1 like several nearby primes.
- Odd
- Composite
Where 91 fits
Why 91 is special
91 is a classic reminder that a number's final digit alone never determines primality — it looks prime at a glance but isn't.
91 is part of
Factors & digit properties
- Divisors (4)
- 1, 7, 13, 91
- Prime factorisation
- 7 × 13
- Digit sum
- 10
- Digital root
- 1
Representations
- Decimal
- 91
- Binary
- 1011011
- Octal
- 133
- Hexadecimal
- 5b
- Roman numeral
- XCI
- Scientific notation
- 9.100 × 10^1
Other mathematical patterns
- Triangular number
- Deficient number
- Happy number
- Hexagonal number
- Semiprime
Related numbers & connections
13 is a factor of 91 (13 × 7 = 91).
View 13 →Mathematically related
- has factor13
why?
Strength 0.50, confidence "high".
Science
91 is the atomic number of protactinium.
DocumentedScience
Mathematics
91 factors as 7 × 13, despite ending in the digit 1 like several nearby primes — a reminder that a number's final digit alone never determines primality.
DocumentedMathematics
Common questions
Is 91 a prime number?
No, 91 is not prime.
What are the factors of 91?
The factors of 91 are 1, 7, 13, 91.
What is 91 in Roman numerals?
91 in Roman numerals is XCI.
About this page's data
Mathematical facts:computed directly from the number's value — nothing to source or verify externally.
Contextual records: 2 reviewer-confirmed records, sourced and reviewed before publication.
Last reviewed: 15 July 2026