561
561 is the smallest Carmichael number — a composite that passes Fermat's primality test for every base.
- Odd
- Composite
Where 561 fits
Why 561 is special
Carmichael numbers are why fast primality tests must be probabilistic and repeated rather than trusted once. 561 factors as 3 x 11 x 17 and still behaves like a prime under the test, which is precisely the failure mode cryptographic software has to defend against.
561 is part of
Factors & digit properties
- Divisors (8)
- 1, 3, 11, 17, 33, 51, 187, 561
- Prime factorisation
- 3 × 11 × 17
- Digit sum
- 12
- Digital root
- 3
Representations
- Decimal
- 561
- Binary
- 1000110001
- Octal
- 1061
- Hexadecimal
- 231
- Roman numeral
- DLXI
- Scientific notation
- 5.610 × 10^2
Other mathematical patterns
- Triangular number
- Deficient number
- Hexagonal number
Mathematics
561 is the smallest Carmichael number: it factors as 3 x 11 x 17 and still satisfies Fermat's primality test for every base coprime to it. Such numbers are why practical primality testing is probabilistic and repeated rather than trusted once, which matters directly to key generation.
DocumentedMathematics
Common questions
Is 561 a prime number?
No, 561 is not prime.
What are the factors of 561?
The factors of 561 are 1, 3, 11, 17, 33, 51, 187, 561.
What is 561 in Roman numerals?
561 in Roman numerals is DLXI.
About this page's data
Mathematical facts:computed directly from the number's value — nothing to source or verify externally.
Contextual records: 1 reviewer-confirmed record, sourced and reviewed before publication.
Last reviewed: 3 August 2026