Definition:Divisor Function

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Definition

Let $\alpha \in \Z_{\ge 0}$ be a non-negative integer.

A divisor function is an arithmetic function of the form:

$\ds \map {\sigma_\alpha} n = \sum_{m \mathop \divides n} m^\alpha$

where the summation is taken over all $m \le n$ such that $m$ divides $n$).


There exist the following special cases:


Divisor Count Function

Let $n$ be an integer such that $n \ge 1$.

The divisor count function is defined on $n$ as being the total number of positive integer divisors of $n$.

It is denoted on $\mathsf{Pr} \infty \mathsf{fWiki}$ as $\sigma_0$ (the Greek letter sigma).

That is:

$\ds \map {\sigma_0} n = \sum_{d \mathop \divides n} 1$

where $\ds \sum_{d \mathop \divides n}$ is the sum over all divisors of $n$.


Divisor Sum Function

Let $n$ be an integer such that $n \ge 1$.

The divisor sum function $\map {\sigma_1} n$ is defined on $n$ as being the sum of all the positive integer divisors of $n$.

That is:

$\ds \map {\sigma_1} n = \sum_{d \mathop \divides n} d$

where $\ds \sum_{d \mathop \divides n}$ is the sum over all divisors of $n$.


Also known as

A divisor function is often seen referred to as a sigma ($\sigma$) function.


Also see

  • Results about divisor functions can be found here.


Sources