# Definition:Tau Function

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## Contents

## Definition

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

The **$\tau$ (tau) function** is defined on $n$ as being the total number of positive integer divisors of $n$.

That is:

- $\displaystyle \map \tau n = \sum_{d \mathop \divides n} 1$

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

## Also known as

Some sources refer to this as the **divisor function** and denote it $\map d n$, but as there is a more general definition of the divisor function the more precise name **tau function** is preferred.

It can also be referred to as the **divisor counting function**.

## Examples

### $\tau$ of $1$

The value of the $\tau$ function for the integer $1$ is $1$.

### $\tau$ of $3$

- $\tau \left({3}\right) = 2$

### $\tau$ of $12$

- $\tau \left({12}\right) = 6$

### $\tau$ of $60$

- $\tau \left({60}\right) = 12$

### $\tau$ of $105$

- $\map \tau {105} = 8$

### $\tau$ of $108$

- $\tau \left({108}\right) = 12$

### $\tau$ of $110$

- $\tau \left({110}\right) = 8$

### $\tau$ of $120$

- $\tau \left({120}\right) = 16$

## Also see

- Results about
**the $\tau$ function**can be found here.

## Linguistic Note

The name of the Greek letter $\tau$ (tau) is properly pronounced to rhyme with **cow** not **caw**.

## Sources

- 1986: David Wells:
*Curious and Interesting Numbers*... (previous) ... (next): Glossary - 1997: David Wells:
*Curious and Interesting Numbers*(2nd ed.) ... (previous) ... (next): Glossary