91
XCI
1011011
The number 91 — factors and divisors
Deficient
91 = 7 × 13. It has 4 divisors, adding up to 112.
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 7 × 13. The more divisors, the closer to a square.
- Prime factors
- 7 × 13
- Divisors
- 1, 7, 13, 91
- Sum of divisors
- 112
- Sum without itself
- 21
- Coprime below it
- 72
- Digit sum
- 10
- Digital root
- 1
- Roman numeral
- XCI
- Binary
- 1011011
- Octal
- 133
- Hexadecimal
- 5B
- Base 36
- 2J
- Bits
- 7 bits
- Collatz steps
- 92 steps · peaks at 9,232
- Prime before
- 89
- Prime after
- 97
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 91 a prime number?
No. It breaks down into 7 × 13.
Q. What are the divisors of 91?
1, 7, 13, 91 — 4 divisors in all, adding up to 112.
Q. How is 91 written in binary?
1011011, which is 7 bits. In hexadecimal it is 5B.
Q. How many Collatz steps does 91 take?
It reaches 1 in 92, peaking at 9,232 on the way.