15
XV
1111
The number 15 — factors and divisors
Deficient
15 = 3 × 5. It has 4 divisors, adding up to 24.
Its divisors, itself excluded, add up to less than the number.
Families it belongs to
Triangular
The running total of 1, 2, 3 … — the dots stack into a triangle.
Laid out as dots
It fits as 3 × 5. The more divisors, the closer to a square.
- Prime factors
- 3 × 5
- Divisors
- 1, 3, 5, 15
- Sum of divisors
- 24
- Sum without itself
- 9
- Coprime below it
- 8
- Digit sum
- 6
- Digital root
- 6
- Roman numeral
- XV
- Binary
- 1111
- Octal
- 17
- Hexadecimal
- F
- Base 36
- F
- Bits
- 4 bits
- Collatz steps
- 17 steps · peaks at 160
- Prime before
- 13
- Prime after
- 17
The grid
Ten per row, twenty rows. The filled cells are primes — you can see the multiples of 2 and 5 drop out in columns.
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Numbers either side
How to read this
- The prime factors are what the number is made of. The count and sum of divisors both follow from them.
- Add the divisors, take the number away, and you get perfect, abundant or deficient.
- The length of the binary form is the bit count. 255 taking eight digits is what a byte is.
- Collatz: halve it if even, triple it and add one if odd. The count is how many moves reach 1.
Frequently asked questions
Q. Is 15 a prime number?
No. It breaks down into 3 × 5.
Q. What are the divisors of 15?
1, 3, 5, 15 — 4 divisors in all, adding up to 24.
Q. How is 15 written in binary?
1111, which is 4 bits. In hexadecimal it is F.
Q. How many Collatz steps does 15 take?
It reaches 1 in 17, peaking at 160 on the way.