Graph (topology)

In topology, a branch of mathematics, a graph is a topological space which arises from a usual graph by replacing vertices by points and each edge by a copy of the unit interval, where is identified with the point associated to and with the point associated to. 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
under the quotient map used for gluing. Here is the 0-skeleton, are the closed intervals glued to it, one for each edge, and is the disjoint union.
The topology on this space is called the graph topology.

Subgraphs and trees

A subgraph of a graph is a subspace which is also a graph and whose nodes are all contained in the 0-skeleton of. is a subgraph if and only if it consists of vertices and edges from and is closed.
A subgraph is called a tree if it is contractible as a topological space. This can be shown equivalent to the usual definition of a tree in graph theory, namely a connected graph without cycles.

The associated topological space of a graph is connected if and only if the original graph is connected.
Every connected graph contains at least one maximal tree, that is, a tree that is maximal with respect to the order induced by set inclusion on the subgraphs of which are trees.
If is a graph and a maximal tree, then the fundamental group equals the free group generated by elements, where the correspond bijectively to the edges of ; in fact, 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.
Every covering space projecting to a graph is also a graph.