What are the divisors of 1285?

1, 5, 257, 1285

There is a total of 4 positive divisors.
The sum of these divisors is 1548.
The arithmetic mean is 387.

4 odd divisors

1, 5, 257, 1285

How to compute the divisors of 1285?

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 1285 by each of the numbers from 1 to 1285 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)

1285 / 1 = 1285 (the remainder is 0, so 1 is a divisor of 1285)
1285 / 2 = 642.5 (the remainder is 1, so 2 is not a divisor of 1285)
1285 / 3 = 428.33333333333 (the remainder is 1, so 3 is not a divisor of 1285)
...
1285 / 1284 = 1.0007788161994 (the remainder is 1, so 1284 is not a divisor of 1285)
1285 / 1285 = 1 (the remainder is 0, so 1285 is a divisor of 1285)

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 1285 (i.e. 35.84689665787). 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 $\frac{N}{d} = D$ )

1285 / 1 = 1285 (the remainder is 0, so 1 and 1285 are divisors of 1285)
1285 / 2 = 642.5 (the remainder is 1, so 2 is not a divisor of 1285)
1285 / 3 = 428.33333333333 (the remainder is 1, so 3 is not a divisor of 1285)
...
1285 / 34 = 37.794117647059 (the remainder is 27, so 34 is not a divisor of 1285)
1285 / 35 = 36.714285714286 (the remainder is 25, so 35 is not a divisor of 1285)