Definition: Null Graph
A simple undirected graph \(G(V,E)\) with \(V=n\) vertices is called a null graph, if it has no edges.
The null graph with \(n\) vertices is denoted by \(N_n\).
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References
Bibliography
 Aldous Joan M., Wilson Robin J.: "Graphs and Applications  An Introductory Approach", Springer, 2000