# Is a single node graph a strongly connected component?

Can a single node graph be considered a (strongly) connected component?

I'm confused because I was reading about cut vertex which by definition is a vertex that if eliminated increases the number of connected components.

So if we were to remove vertex number 2, we would get 2 connected components?

Thanks!

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## 2 Answers

A graph is connected if every pair of vertices in the graph can be connected by a path. By definition a single vertex is connected to itself by the trivial path. Hence, it is connected. If the graph is directed the same argument applies.

See also http://mathworld.wolfram.com/ConnectedGraph.html, specifically:

A graph which is connected in the sense of a topological space, i.e., there is a path from any point to any other point in the graph. A graph that is not connected is said to be disconnected. This definition means that the null graph and singleton graph are considered connected, while empty graphs on n>=2 nodes are disconnected.

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This is a bad definition of connectivity: the correct definition should be that a connected graph is a graph with one connected component (connected components are the equivalence classes of the equivalence relation which is the transitive closure of the relation given by the edges). By this definition the empty graph is disconnected. See math.stackexchange.com/questions/50551/… . –  Qiaochu Yuan May 22 '12 at 17:54
@QiaochuYuan: hmm, yes, I haven't really thought about connectedness from a graph-theoretical (rather than a topological) point of view, but you are of course right. Nonetheless, I feel the post as it stands answers OP's question(s). –  M.B. May 22 '12 at 18:13

Reflexive property: For all a, a # a. Any vertex is strongly connected to itself, by definition. Source: https://www.ics.uci.edu/~eppstein/161/960220.html

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