What are the divisors of 1649?

1, 17, 97, 1649

There is a total of 4 positive divisors.
The sum of these divisors is 1764.
The arithmetic mean is 441.

4 odd divisors

1, 17, 97, 1649

How to compute the divisors of 1649?

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 1649 by each of the numbers from 1 to 1649 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)

1649 / 1 = 1649 (the remainder is 0, so 1 is a divisor of 1649)
1649 / 2 = 824.5 (the remainder is 1, so 2 is not a divisor of 1649)
1649 / 3 = 549.66666666667 (the remainder is 2, so 3 is not a divisor of 1649)
...
1649 / 1648 = 1.0006067961165 (the remainder is 1, so 1648 is not a divisor of 1649)
1649 / 1649 = 1 (the remainder is 0, so 1649 is a divisor of 1649)

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 1649 (i.e. 40.607881008494). 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 $\frac{N}{d} = D$ )

1649 / 1 = 1649 (the remainder is 0, so 1 and 1649 are divisors of 1649)
1649 / 2 = 824.5 (the remainder is 1, so 2 is not a divisor of 1649)
1649 / 3 = 549.66666666667 (the remainder is 2, so 3 is not a divisor of 1649)
...
1649 / 39 = 42.282051282051 (the remainder is 11, so 39 is not a divisor of 1649)
1649 / 40 = 41.225 (the remainder is 9, so 40 is not a divisor of 1649)