subspace (topology)

In topology and related areas of mathematics, a subspace of a topological space (''X'', ''𝜏'') is a subset ''S'' of ''X'' which is equipped with a topology induced from that of ''𝜏'' called the subspace topology (or the relative topology, or the induced topology, or the trace topology).; see Section 26.2.4. Submanifolds, p. 59

Definition

Given a topological space $(X, \tau)$ and a subset $S$ of $X$, the subspace topology on $S$ is defined by
:$\tau_S = \lbrace S \cap U \mid U \in \tau \rbrace.$
That is, a subset of $S$ is open in the subspace topology if and only if it is the intersection of $S$ with an open set in $(X, \tau)$. If $S$ is equipped with the subspace topology then it is a topological space in its own right, and is called a subspace of $(X, \tau)$. Subsets of topological spaces are usually assumed to be equipped with the subspace topology unless otherwise stated.

Alternatively we can define the subspace topology for a subset $S$ of $X$ as the coarsest topology for which the inclusion map
:$\iota: S \hookrightarrow X$
is continuous.

More generally, suppose $\iota$ is an injection from a set $S$ to a topological space $X$. Then the subspace topology on $S$ is defined as the coarsest topology for which $\iota$ is continuous. The open sets in this topology are precisely the ones of the form $\iota^(U)$ for $U$ open in $X$. $S$ is then homeomorphic to its image in $X$ (also with the subspace topology) and $\iota$ is called a topological embedding.

A subspace $S$ is called an open subspace if the injection $\iota$ is an open map, i.e., if the forward image of an open set of $S$ is open in $X$. Likewise it is called a closed subspace if the injection $\iota$ is a closed map.

Terminology

The distinction between a set and a topological space is often blurred notationally, for convenience, which can be a source of confusion when one first encounters these definitions. Thus, whenever $S$ is a subset of $X$, and $(X, \tau)$ is a topological space, then the unadorned symbols "$S$" and "$X$" can often be used to refer both to $S$ and $X$ considered as two subsets of $X$, and also to $(S,\tau_S)$ and $(X,\tau)$ as the topological spaces, related as discussed above. So phrases such as "$S$ an open subspace of $X$" are used to mean that $(S,\tau_S)$ is an open subspace of $(X,\tau)$, in the sense used above; that is: (i) $S \in \tau$; and (ii) $S$ is considered to be endowed with the subspace topology.

Examples

In the following, $\mathbb$ represents the real numbers with their usual topology.
* The subspace topology of the natural numbers, as a subspace of $\mathbb$, is the discrete topology.
* The rational numbers $\mathbb$ considered as a subspace of $\mathbb$ do not have the discrete topology ( for example is not an open set in $\mathbb$ because there is no open subset of $\mathbb$ whose intersection with $\mathbb$ can result in ''only'' the singleton ). If ''a'' and ''b'' are rational, then the intervals (''a'', ''b'') and 'a'', ''b''are respectively open and closed, but if ''a'' and ''b'' are irrational, then the set of all rational ''x'' with ''a'' < ''x'' < ''b'' is both open and closed.
* The set ,1as a subspace of $\mathbb$ is both open and closed, whereas as a subset of $\mathbb$ it is only closed.
* As a subspace of $\mathbb$, , 1∪ , 3is composed of two disjoint ''open'' subsets (which happen also to be closed), and is therefore a disconnected space.
* Let ''S'' = if and only if