# Definition:Composite Number

## Contents

## Definition

A **composite number** $c$ is a positive integer that has more than two positive divisors.

That is, an integer greater than $1$ which is not prime is defined as composite.

In the words of Euclid:

*A***composite number**is that which is measured by some number.

(*The Elements*: Book $\text{VII}$: Definition $13$)

## Extension to Negative Integers

The definition of a **composite number** can be extended to the negative integers, as follows:

A negative integer $n$ is composite if and only if $\left|{n}\right|$ is composite.

## Special Cases

### Zero

$0$ is not considered **composite**.

$1$ is also a special case - it is neither prime nor **composite**.

All of the other positive integers are either prime or **composite**.

### Plane Number

A **plane number** is the product of two (natural) numbers.

In the words of Euclid:

*And, when two numbers having multiplied one another make some number, the number so produced is called***plane**, and its**sides**are the numbers which have multiplied one another.

(*The Elements*: Book $\text{VII}$: Definition $16$)

### Solid Number

A **solid number** is the product of three (natural) numbers.

In the words of Euclid:

*And, when three numbers having multiplied one another make some number, the number so produced is***solid**, and its**sides**are the numbers which have multiplied one another.

(*The Elements*: Book $\text{VII}$: Definition $17$)

## Also known as

Some sources refer to a **composite number** as a **factorable** or **factorizable** number.

For many reasons, one being that **composite** is of fewer syllables and therefore more economical to say, **composite** is the preferred form on $\mathsf{Pr} \infty \mathsf{fWiki}$.

## Historical Note

The concept of classifying numbers as **prime** or **composite** appears to have originated with the Pythagoreans.

## Sources

- 1978: Thomas A. Whitelaw:
*An Introduction to Abstract Algebra*... (previous) ... (next): $\S 12$: Highest common factors and Euclid's algorithm - 1986: David Wells:
*Curious and Interesting Numbers*... (previous) ... (next): Glossary - 1992: George F. Simmons:
*Calculus Gems*... (previous) ... (next): Chapter $\text {B}.16$: The Sequence of Primes - 1997: David Wells:
*Curious and Interesting Numbers*(2nd ed.) ... (previous) ... (next): Glossary - 2008: David Joyner:
*Adventures in Group Theory*(2nd ed.) ... (previous) ... (next): $\S 2.1$: Functions: Example $2.1.1$ - 2008: Ian Stewart:
*Taming the Infinite*... (previous) ... (next): Chapter $7$: Patterns in Numbers