In mathematics, a graph is used to show how things are connected. The mathematical study on graph is called graph theory. The things being connected are called vertices, and the connections among them are called edges. If vertices are connected by an edge, they are called adjacent. The degree of a vertex is the number of edges that connect to it. A graph's size formula_1is the number of edges in total. The number of vertices, written as formula_2, is called the order of a graph. 