Graph (topology)

In topology, a subject in mathematics, a graph is a topological space which arises from a usual graph $G=(E,V)$ by replacing vertices by points and each edge $e=xy\in E$ by a copy of the unit interval $[0,1]$ where $0$ is identified with the point associated to $x$ and $1$ with the point associated to $y$ . That is, as topological spaces, graphs are exactly the simplicial 1-complexes and also exactly the one-dimensional CW complexes.

Thus, in particular, it bears the quotient topology of the set

$X_{0}\sqcup \bigsqcup _{e\in E}I_{e}$ (disjoint union)

under the quotient map used for gluing, where $X_{0}$ is the 0-skeleton (consisting one point for each vertex $x\in V$ ) and $I_{e}$ are the intervals ("closed one-dimensional unit balls") glued to it, one for each edge $e\in E$ .

The topology on this space is called the graph topology.

Subgraphs and -trees

A subgraph of a graph $X$ is a subspace $Y\subseteq X$ which is also a graph and whose nodes are all contained in the 0-skeleton of $X$ . $Y$ is a subgraph if and only if it consists of vertices and edges from $X$ and is closed.

A subgraph $T\subseteq X$ is called a tree iff it is contractible as a topological space.

Properties

• Every connected graph $X$ contains a maximal tree $T\subseteq X$ , that is, a tree that is maximal with respect to the order induced by set inclusion on the subgraphs of $X$ which are trees.
• If $X$ is a graph and $T\subseteq X$ a maximal tree, then the fundamental group $\pi _{1}(X)$ equals the free group generated by elements $(f_{\alpha })_{\alpha \in A}$ , where the $\{f_{\alpha }\}$ correspond bijectively to the edges of $X\setminus T$ ; in fact, $X$ is homotopy equivalent to a wedge sum of circles.
• Forming the topological space associated to a graph as above amounts to a functor from the category of graphs to the category of topological spaces.
• The associated topological space of a graph is connected (with respect to the graph topology) if and only if the original graph is connected.
• Every covering space projecting to a graph is also a graph.

Applications

Using the above properties of graphs, one can prove the Nielsen–Schreier theorem.