What are the divisors of 91?

1, 7, 13, 91

There is a total of 4 positive divisors.
The sum of these divisors is 112.
The arithmetic mean is 28.

4 odd divisors

1, 7, 13, 91

How to compute the divisors of 91?

A number N is said to be divisible by a number M (with M non-zero) if, when we divide N by M, the remainder of the division is zero.

$N mod M = 0$

Brute force algorithm

We could start by using a brute-force method which would involve dividing 91 by each of the numbers from 1 to 91 to determine which ones have a remainder equal to 0.

$Remainder = N - (M \times ⌊ \frac{N}{M} ⌋)$

(where $⌊ \frac{N}{M} ⌋$ is the integer part of the quotient)

91 / 1 = 91 (the remainder is 0, so 1 is a divisor of 91)
91 / 2 = 45.5 (the remainder is 1, so 2 is not a divisor of 91)
91 / 3 = 30.333333333333 (the remainder is 1, so 3 is not a divisor of 91)
...
91 / 90 = 1.0111111111111 (the remainder is 1, so 90 is not a divisor of 91)
91 / 91 = 1 (the remainder is 0, so 91 is a divisor of 91)

Improved algorithm using square-root

However, there is another slightly better approach that reduces the number of iterations by testing only integers less than or equal to the square root of 91 (i.e. 9.5393920141695). Indeed, if a number N has a divisor D greater than its square root, then there is necessarily a smaller divisor d such that:

$D \times d = N$

(thus, if $\frac{N}{D} = d$ , then $\frac{N}{d} = D$ )

91 / 1 = 91 (the remainder is 0, so 1 and 91 are divisors of 91)
91 / 2 = 45.5 (the remainder is 1, so 2 is not a divisor of 91)
91 / 3 = 30.333333333333 (the remainder is 1, so 3 is not a divisor of 91)
...
91 / 8 = 11.375 (the remainder is 3, so 8 is not a divisor of 91)
91 / 9 = 10.111111111111 (the remainder is 1, so 9 is not a divisor of 91)