如何定义图这个数学概念？ What we have drawn in Figure 1.1 is called a graph.Formally,a graph G consists of a finite nonempty set V of objects called vertices (the singular is vertex)and a set E of 2- element subsets of V called edges.The sets V and E are the vertex set and edge set of G, G=V,E) E={{u,vu,V∈V如何定义图这个数学概念？ G = (V ,E ) E = {{u,v}| u,v  V }