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,vV(G),G has exactly one u,v-path. 问题2： 如何证明一系列命题等价？ 问题38 不能有回路对最小顶点度数有什么影响？