# Difference between revisions of "Metric space"

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− | A '''metric space''' is a pair, <math>(S, d)</math> of a [[set]] <math>S</math> and a [[metric]] <math>d: S \times S \to \mathbb{R}_{\geq 0}</math>. The metric <math>d</math> represents a distance function between pairs of points of <math>S</math> | + | A '''metric space''' is a pair, <math>(S, d)</math> of a [[set]] <math>S</math> and a [[metric]] <math>d: S \times S \to \mathbb{R}_{\geq 0}</math>. The metric <math>d</math> represents a distance function between pairs of points of <math>S</math> which has the following properties: |

+ | |||

+ | *Symmetry (<math>d(x, y) = d(y, x)</math>) | ||

+ | *Non-negativity (<math>d(x, y) \geq 0</math> | ||

+ | *Uniqueness (<math>d(x, y) = 0</math> if and only if <math>x = y</math>) | ||

+ | *[[Triangle Inequality]] (<math>d(x, y) + d(y, z) \geq d(x, z)</math> for all points <math>x, y, z \in S</math>). | ||

+ | |||

+ | Intuitively, a metric space is a generalization of the distance between two objects (where "objects" can be anything, including points, functions, graphics, or grades). The above properties follow from our notion of distance. Non-negativity stems from the idea that A cannot be closer to B than B is to itself; Uniqueness results from two objects being identical if and only if they are the same object; and the Triangle Inequality corresponds to the idea that a direct path between points A and B should be at least as short as a roundabout path that visits some point C first. | ||

==Popular metrics== | ==Popular metrics== | ||

− | * The [[Euclidean | + | * The [[Euclidean Metric]] on <math>\mathbb{R}^n</math>, with the "usual" meaning of distance |

− | * The [[ | + | * The [[Discrete Metric]] on any set |

{{stub}} | {{stub}} |

## Revision as of 22:42, 29 November 2006

A **metric space** is a pair, of a set and a metric . The metric represents a distance function between pairs of points of which has the following properties:

- Symmetry ()
- Non-negativity (
- Uniqueness ( if and only if )
- Triangle Inequality ( for all points ).

Intuitively, a metric space is a generalization of the distance between two objects (where "objects" can be anything, including points, functions, graphics, or grades). The above properties follow from our notion of distance. Non-negativity stems from the idea that A cannot be closer to B than B is to itself; Uniqueness results from two objects being identical if and only if they are the same object; and the Triangle Inequality corresponds to the idea that a direct path between points A and B should be at least as short as a roundabout path that visits some point C first.

## Popular metrics

- The Euclidean Metric on , with the "usual" meaning of distance

- The Discrete Metric on any set

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