Complete graph
Look familiar?
A complete graph, or mystic rose, is an undirected graph where every vertex is connected to every other vertex. The symbol \(K_n\) denotes the complete graph with \(n\) vertices.
The number of edges in \(K_n\) is always exactly \(\binom{n}{2}.\) This is the total number of ways that \(2\) vertices can be chosen to make an edge. Since every possible edge is present in \(K_n\), counting them is the same as counting every single way of making an edge.
An empty graph is considered the counterpart to a complete graph.