For an n-vertex graph G(with n 1),the following are equiv- alent(and characterize the trees with n vertices). A)G is connected and has no cycles. B)G is connected and has n-1 edges. C)G has n-1 edges and no cycles. D)For u,vE V(G),G has exactly one 4,v-path 问题2： 如何证明一系列命题等价？ 问题48 不能有回路对最小顶点度 数有什么影响？ 问题3：有n-1条边的n点连通图，一定是树？ 有n-1条边的无环连通图，一定连通n个点？问题3：有n-1条边的n点连通图，一定是树？ 有n-1条边的无环连通图，一定连通n个点？