# Definition:Distance/Sets/Real Numbers

## Definition

Let $S, T$ be a subsets of the set of real numbers $\R$.

Let $x \in \R$ be a real number.

The distance between $x$ and $S$ is defined and annotated $\displaystyle \map d {x, S} = \map {\inf_{y \mathop \in S} } {\map d {x, y} }$, where $\map d {x, y}$ is the distance between $x$ and $y$.

The distance between $S$ and $T$ is defined and annotated $\displaystyle \map d {S, T} = \map {\inf_{\substack {x \mathop \in S \\ y \mathop \in T} } } {\map d {x, y} }$.

## Also denoted as

Some sources write $\operatorname{dist}$ instead of $d$.

## Examples

### Example 1

Let $S \subseteq \R$ be the subset of the set of real numbers $\R$ defined as:

$S := \set {0, 1, 2}$

Then:

$\map d {3, S} = 1$

### Example 2

Let $S \subseteq \R$ be the subset of the set of real numbers $\R$ defined as:

$S := \openint 0 1$

Then:

$\map d {3, S} = 1$

### Example 3

Let $S \subseteq \R$ be the subset of the set of real numbers $\R$ defined as:

$S := \closedint 1 2$

Then:

$\map d {3, S} = 1$

### Example 4

Let $S \subseteq \R$ be the subset of the set of real numbers $\R$ defined as:

$S := \openint 2 3$

Then:

$\map d {3, S} = 0$