Number analysis
15
15 is a composite odd number that factors as 3 × 5.
15 is a composite odd number that factors as 3 × 5.
The number itself
- Value
- 15
- Digits
- 2
- Digit sum
- 6
- Reversed
- 51
- Palindrome
- No
1 + 5
Digit sum repeated until one digit remains; equals n mod 9 (with 9 for multiples of 9).
Reverses to 51.
Parity and primality
- Parity
- Odd
- Prime
- No
- Prime factorisation
- 3 × 5
- Squarefree
- Yes
- Semiprime
- Yes
- Möbius μ(n)
- 1
- Nearest primes
- 13 ← → 17
Leaves remainder 1 on division by 2.
Composite — it factors as 3 × 5.
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 4 between them.
Divisors
- Aliquot sum
- 9
- Classification
- Deficient
1, 3, 5, 15
The proper divisors — everything except n itself.
Its proper divisors fall short by 6.
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
- No
- Sum of four squares
- Yes
A prime congruent to 3 mod 4 divides it an odd number of times, which rules it out.
Excluded by Legendre: it has the form 4^a(8b + 7).
Lagrange's four-square theorem: every non-negative integer is, without exception.
Shapes and sequences
- Triangular number
- Yes — the 5th
- Hexagonal number
- Yes — the 3rd
Digit curiosities
- Happy number
- No
- Harshad number
- No
- Collatz trajectory
- 17 steps to 1
Summing the squares of its digits falls into the 4 → 16 → 37 → … → 4 cycle.
Not divisible by its digit sum 6.
Peaks at 160 along the way.
Written other ways
- Binary
- 1111
- Octal
- 17
- Base 36
- F
- In words
- fifteen
- Scientific notation
- 1.5 × 10^1
Is 15 a prime number?
Composite — it factors as 3 × 5.
Nearby numbers
Other numeral systems
Beyond the bases and Roman numerals above, these notations write 15 their own way.
- Greek (alphabetic)
- ΙΕʹ
- Babylonian cuneiform
- 𒌋𒐙
- Tamil
- ௰௫
References
Further reading: Wikipedia · MathWorld · 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.