Interior (topology)

right|thumb|The point is an interior point of . The point is on the boundary of .

In mathematics, specifically in topology,

the interior of a subset of a topological space is the union of all subsets of that are open in .

A point that is in the interior of is an interior point of .

The interior of is the complement of the closure of the complement of .

In this sense interior and closure are dual notions.

The exterior of a set is the complement of the closure of ; it consists of the points that are in neither the set nor its boundary.

The interior, boundary, and exterior of a subset together partition the whole space into three blocks (or fewer when one or more of these is empty).

Definitions

Interior point

If <math>S</math> is a subset of a Euclidean space, then <math>x</math> is an interior point of <math>S</math> if there exists an open ball centered at <math>x</math> which is completely contained in <math>S.</math>

(This is illustrated in the introductory section to this article.)

This definition generalizes to any subset <math>S</math> of a metric space <math>X</math> with metric <math>d</math>: <math>x</math> is an interior point of <math>S</math> if there exists a real number <math>r > 0,</math> such that <math>y</math> is in <math>S</math> whenever the distance <math>d(x, y) < r.</math>

This definition generalizes to topological spaces by replacing "open ball" with "open set".

If <math>S</math> is a subset of a topological space <math>X</math> then <math>x</math> is an of <math>S</math> in <math>X</math> if <math>x</math> is contained in an open subset of <math>X</math> that is completely contained in <math>S.</math>

(Equivalently, <math>x</math> is an interior point of <math>S</math> if <math>S</math> is a neighbourhood of <math>x.</math>)

Interior of a set

The interior of a subset <math>S</math> of a topological space <math>X,</math> denoted by <math>\operatorname{int}_X S</math> or <math>\operatorname{int} S</math> or <math>S^\circ,</math> can be defined in any of the following equivalent ways:

<math>\operatorname{int} S</math> is the largest open subset of <math>X</math> contained in <math>S.</math>
<math>\operatorname{int} S</math> is the union of all open sets of <math>X</math> contained in <math>S.</math>
<math>\operatorname{int} S</math> is the set of all interior points of <math>S.</math>

If the space <math>X</math> is understood from context then the shorter notation <math>\operatorname{int} S</math> is usually preferred to <math>\operatorname{int}_X S.</math>

Examples

thumb|<math>a</math> is an interior point of <math>M</math> because there is an ε-neighbourhood of <math>a</math> which is a subset of <math>M.</math>

In any space, the interior of the empty set is the empty set.
In any space <math>X,</math> if <math>S \subseteq X,</math> then <math>\operatorname{int} S \subseteq S.</math>
If <math>X</math> is the real line <math>\Reals</math> (with the standard topology), then <math>\operatorname{int} ([0, 1]) = (0, 1)</math> whereas the interior of the set <math>\Q</math> of rational numbers is empty: <math>\operatorname{int} \Q = \varnothing.</math>
If <math>X</math> is the complex plane <math>\Complex,</math> then <math>\operatorname{int} (\{z \in \Complex : |z| \leq 1\}) = \{z \in \Complex : |z| < 1\}.</math>
In any Euclidean space, the interior of any finite set is the empty set.

On the set of real numbers, one can put other topologies rather than the standard one:

If <math>X</math> is the real numbers <math>\Reals</math> with the lower limit topology, then <math>\operatorname{int} ([0, 1]) = [0, 1).</math>
If one considers on <math>\Reals</math> the topology in which every set is open, then <math>\operatorname{int} ([0, 1]) = [0, 1].</math>
If one considers on <math>\Reals</math> the topology in which the only open sets are the empty set and <math>\Reals</math> itself, then <math>\operatorname{int} ([0, 1])</math> is the empty set.

These examples show that the interior of a set depends upon the topology of the underlying space.

The last two examples are special cases of the following.

In any discrete space, since every set is open, every set is equal to its interior.
In any indiscrete space <math>X,</math> since the only open sets are the empty set and <math>X</math> itself, <math>\operatorname{int} X = X</math> and for every proper subset <math>S</math> of <math>X,</math> <math>\operatorname{int} S</math> is the empty set.

Properties

Let <math>X</math> be a topological space and let <math>S</math> and <math>T</math> be subsets of <math>X.</math>

<math>\operatorname{int} S</math> is open in <math>X.</math>
If <math>T</math> is open in <math>X</math> then <math>T \subseteq S</math> if and only if <math>T \subseteq \operatorname{int} S.</math>
<math>\operatorname{int} S</math> is an open subset of <math>S</math> when <math>S</math> is given the subspace topology.
<math>S</math> is an open subset of <math>X</math> if and only if <math>\operatorname{int} S = S.</math>
: <math>\operatorname{int} S \subseteq S.</math>
Idempotent|: <math>\operatorname{int} (\operatorname{int} S) = \operatorname{int} S.</math>
/: <math>\operatorname{int} (S \cap T) = (\operatorname{int} S) \cap (\operatorname{int} T).</math>
However, the interior operator does not distribute over unions since only <math>\operatorname{int} (S \cup T) ~\supseteq~ (\operatorname{int} S) \cup (\operatorname{int} T)</math> is guaranteed in general and equality might not hold. |note=C. Ursescu|math_statement=

Let <math>S_1, S_2, \ldots</math> be a sequence of subsets of a complete metric space <math>X.</math>

If each <math>S_i</math> is closed in <math>X</math> then <math display="block">

{\operatorname{cl}_X} \biggl( \bigcup_{i \in \N} \operatorname{int}_X S_i \biggr)

= {\operatorname{cl}_X \operatorname{int}_X} \biggl( \bigcup_{i \in \N} S_i \biggr).

</math>

If each <math>S_i</math> is open in <math>X</math> then <math display="block">

{\operatorname{int}_X} \biggl( \bigcap_{i \in \N} \operatorname{cl}_X S_i \biggr)

= {\operatorname{int}_X \operatorname{cl}_X} \biggl( \bigcap_{i \in \N} S_i \biggr).

</math>

The result above implies that every complete metric space is a Baire space.

Exterior of a set

The exterior of a subset <math>S</math> of a topological space <math>X,</math> denoted by <math>\operatorname{ext}_X S</math> or simply <math>\operatorname{ext} S,</math> is the largest open set disjoint from <math>S,</math> namely, it is the union of all open sets in <math>X</math> that are disjoint from <math>S.</math> The exterior is the interior of the complement, which is the same as the complement of the closure; in formulas,

<math display="block">\operatorname{ext} S = \operatorname{int}(X \setminus S) = X \setminus \overline{S}.</math>

Similarly, the interior is the exterior of the complement:

<math display="block">\operatorname{int} S = \operatorname{ext}(X \setminus S).</math>

The interior, boundary, and exterior of a set <math>S</math> together partition the whole space into three blocks (or fewer when one or more of these is empty):

<math display="block">X = \operatorname{int} S \cup \partial S \cup \operatorname{ext} S,</math>

where <math>\partial S</math> denotes the boundary of <math>S.</math> The interior and exterior are always open, while the boundary is closed.

Some of the properties of the exterior operator are unlike those of the interior operator:

The exterior operator reverses inclusions; if <math>S \subseteq T,</math> then <math>\operatorname{ext} T \subseteq \operatorname{ext} S.</math>
The exterior operator is not idempotent. It does have the property that <math>\operatorname{int} S \subseteq \operatorname{ext}\left(\operatorname{ext} S\right).</math>

Interior-disjoint shapes

right|thumb|The red shapes are not interior-disjoint with the blue Triangle. The green and the yellow shapes are interior-disjoint with the blue Triangle, but only the yellow shape is entirely disjoint from the blue Triangle.

Two shapes <math>a</math> and <math>b</math> are called interior-disjoint if the intersection of their interiors is empty.

Interior-disjoint shapes may or may not intersect in their boundary.

References

Bibliography