# Definition:Euclidean Metric/Real Vector Space

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## Definition

Let $\R^n$ be an $n$-dimensional real vector space.

The Euclidean metric on $\R^n$ is defined as:

$\ds \map {d_2} {x, y} := \paren {\sum_{i \mathop = 1}^n \paren {x_i - y_i}^2}^{1 / 2}$

where $x = \tuple {x_1, x_2, \ldots, x_n}, y = \tuple {y_1, y_2, \ldots, y_n} \in \R^n$.

### Real Number Line

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 $x - y$.

### Real Number Plane

The Euclidean metric on $\R^2$ is defined as:

$\ds \map {d_2} {x, y} := \sqrt {\paren {x_1 - y_1}^2 + \paren {x_2 - y_2}^2}$

where $x = \tuple {x_1, x_2}, y = \tuple {y_1, y_2} \in \R^2$.

### Complex Plane

The Euclidean metric on $\C$ is defined as:

$\d \forall z_1, z_2 \in \C: \map d {z_1, z_2} := \size {z_1 - z_2}$

where $\size {z_1 - z_2}$ denotes the modulus of $z_1 - z_2$.

### Ordinary Space

The Euclidean metric on $\R^3$ is defined as:

$\map {d_2} {x, y} := \sqrt {\paren {x_1 - y_1}^2 + \paren {x_2 - y_2}^2 + \paren {x_3 - y_3}^2}$

where $x = \tuple {x_1, x_2, x_3}, y = \tuple {y_1, y_2, y_3} \in \R^3$.

## Also known as

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

## Also see

In this context, the topology induced by the Euclidean metric is sometimes called the usual topology.

• Results about 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.