Some graphs have special properties.
For example, the graphs, are both graphs on five vertices. In the graph on the left, no vertices are connected at all, whereas in the graph on the right, every vertex is joined to every other vertex by exactly one edge. This graph on the left is called the null graph on five vertices, and the graph on the right is called the complete graph on five vertices. Definition A graph whose edge set is empty is called a null graph . A null graph on n vertices is denoted by N n .