# Graph's Vertex Neighbourhood

A Graph's Vertex Neighbourhood is an Induced Subgraph formed by all the other vertices adjacent to it.

**AKA:**Graph Theory Neighbourhood.**Context:**- It can range from being a Open Neighbourhood to being a Closed Neighbourhood.

**See:**Induced Subgraph, Loop (Graph Theory), Neighbourhood (Mathematics), Neighbourhood (Disambiguation), Graph Theory, Vertex (Graph Theory), Graph (Discrete Mathematics), Edge (Graph Theory), Adjacency List, Adjacency Matrix, Clustering Coefficient, Dense Graph.

## References

### 2019

- (Wikipedia, 2019) ⇒ https://en.wikipedia.org/wiki/Neighbourhood_(graph_theory) Retrieved:2019-5-3.
- In graph theory, an
**adjacent vertex**of a vertex*v*in a graph is a vertex that is connected to*v*by an edge. The**neighbourhood**of a vertex*v*in a graph*G*is the subgraph of*G*induced by all vertices adjacent to*v*, i.e., the graph composed of the vertices adjacent to*v*and all edges connecting vertices adjacent to*v*. For example, in the image to the right, the neighbourhood of vertex 5 consists of vertices 1, 2 and 4 and the edge connecting vertices 1 and 2.The neighbourhood is often denoted

*N*_{G}(*v*) or (when the graph is unambiguous)*N*(*v*). The same neighbourhood notation may also be used to refer to sets of adjacent vertices rather than the corresponding induced subgraphs. The neighbourhood described above does not include*v*itself, and is more specifically the**open neighbourhood**of*v*; it is also possible to define a neighbourhood in which*v*itself is included, called the**closed neighbourhood**and denoted by*N*_{G}[*v*]. When stated without any qualification, a neighbourhood is assumed to be open.Neighbourhoods may be used to represent graphs in computer algorithms, via the adjacency list and adjacency matrix representations. Neighbourhoods are also used in the clustering coefficient of a graph, which is a measure of the average density of its neighbourhoods. In addition, many important classes of graphs may be defined by properties of their neighbourhoods, or by symmetries that relate neighbourhoods to each other.

An isolated vertex has no adjacent vertices. The degree of a vertex is equal to the number of adjacent vertices. A special case is a loop that connects a vertex to itself; if such an edge exists, the vertex belongs to its own neighbourhood.

- In graph theory, an