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) 如果要定 E={u,v}u,v∈Vy 义有向图？如何定义图这个数学概念？ G  (V ,E ) E  {{u,v}| u,v  V } 如果要定 义有向图？