The closed neighborhood of a graph vertex 
in a graph 
is the set
![N[v]={v} union {u:uv in E(G)},](/images/equations/ClosedNeighborhood/NumberedEquation1.svg) |
consisting of 
together with every vertex adjacent to 
. It is also called the graph neighborhood
of 
.
Two adjacent vertices are true twins exactly when they have the same closed neighborhood.
