# Definition:Euclidean Metric/Real Number Line

## Definition

Consider the Euclidean space $\struct {\R^n, d}$.

On the **real number line**, the **Euclidean metric** can be seen to degenerate to:

- $\map d {x, y} := \sqrt {\paren {x - y}^2} = \size {x - y}$

where $\size {x - y}$ denotes the absolute value of $x - y$.

## Also known as

The **Euclidean metric** is also known as the **Euclidean distance**.

Some sources call it the **product metric**.

Some sources refer to it as the **Cartesian distance** or **Cartesian metric**, for René Descartes.

The **Euclidean metric** is sometimes also referred to as **the usual metric**.

## Also see

- Results about
**the Euclidean metric**can be found**here**.

- Results about
**Euclidean spaces**can be found**here**.

- Results about
**the real number line with the Euclidean metric**can be found**here**.

## Source of Name

This entry was named for Euclid.

## Historical Note

Euclid himself did not in fact conceive of the Euclidean metric and its associated Euclidean space, Euclidean topology and Euclidean norm.

They bear that name because the geometric space which it gives rise to is **Euclidean** in the sense that it is consistent with Euclid's fifth postulate.

## Sources

- 1967: George McCarty:
*Topology: An Introduction with Application to Topological Groups*... (previous) ... (next): Chapter $\text{III}$: Metric Spaces: The Definition - 1975: Bert Mendelson:
*Introduction to Topology*(3rd ed.) ... (previous) ... (next): Chapter $2$: Metric Spaces: $\S 2$: Metric Spaces - 1978: Lynn Arthur Steen and J. Arthur Seebach, Jr.:
*Counterexamples in Topology*(2nd ed.) ... (previous) ... (next): Part $\text {II}$: Counterexamples: $28$. Euclidean Topology - 1998: David Nelson:
*The Penguin Dictionary of Mathematics*(2nd ed.) ... (previous) ... (next):**topological space** - 1999: Theodore W. Gamelin and Robert Everist Greene:
*Introduction to Topology*(2nd ed.) ... (previous) ... (next):**One**: Metric Spaces: $1$: Open and Closed Sets - 2008: David Nelson:
*The Penguin Dictionary of Mathematics*(4th ed.) ... (previous) ... (next):**topological space**