# Definition:Infinity

## Definition

Informally, the term **infinity** is used to mean **some infinite number**, but this concept falls very far short of a usable definition.

The symbol $\infty$ (supposedly invented by John Wallis) is often used in this context to mean **an infinite number**.

However, outside of its formal use in the definition of limits its use is strongly discouraged until you know what you're talking about.

It is defined as having the following properties:

- $\forall n \in \Z: n < \infty$

- $\forall n \in \Z: n + \infty = \infty$

- $\forall n \in \Z: n \times \infty = \infty$

- $\infty^2 = \infty$

Similarly, the quantity written as $-\infty$ is defined as having the following properties:

- $\forall n \in \Z: -\infty< n$

- $\forall n \in \Z: -\infty + n = -\infty$

- $\forall n \in \Z: -\infty \times n = -\infty$

- $\left({-\infty}\right)^2 = -\infty$

The latter result seems wrong when you think of the rule that a negative number square equals a positive one, but remember that infinity is not exactly a number as such.

## Also known as

The term **ad infinitum** can often be found in early texts. It is Latin for **to infinity**.

## Historical Note

The symbol $\infty$ for infinity was introduced by John Wallis in the $17$th century.

## Also see

- Definition:Extended Real Number Line
- Definition:Extended Natural Number
- Definition:Positive Infinity
- Definition:Negative Infinity
- Definition:Projective Line
- Definition:New Element

## Sources

- 1977: K.G. Binmore:
*Mathematical Analysis: A Straightforward Approach*... (previous) ... (next): $\S 1.3$: Arithmetic