81
LXXXI
1010001
The number 81 — factors and divisors
Deficient
81 = 3⁴. It has 5 divisors, adding up to 121.
Its divisors, itself excluded, add up to less than the number.
Families it belongs to
Square
A number times itself — the dots lay out as a square.
Laid out as dots
It fits as 9 × 9. The more divisors, the closer to a square.
- Prime factors
- 3⁴
- Divisors
- 1, 3, 9, 27, 81
- Sum of divisors
- 121
- Sum without itself
- 40
- Coprime below it
- 54
- Digit sum
- 9
- Digital root
- 9
- Roman numeral
- LXXXI
- Binary
- 1010001
- Octal
- 121
- Hexadecimal
- 51
- Base 36
- 29
- Bits
- 7 bits
- Collatz steps
- 22 steps · peaks at 244
- Prime before
- 79
- Prime after
- 83
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 81 a prime number?
No. It breaks down into 3⁴.
Q. What are the divisors of 81?
1, 3, 9, 27, 81 — 5 divisors in all, adding up to 121.
Q. How is 81 written in binary?
1010001, which is 7 bits. In hexadecimal it is 51.
Q. How many Collatz steps does 81 take?
It reaches 1 in 22, peaking at 244 on the way.