2连通图的特征 4.2.4.Theorem.For a graph G with at least three vertices,the following con- ditions are equivalent (and characterize 2-connected graphs). A)G is connected and has no cut-vertex. B)For all x,y V(G),there are internally disjoint x,y-paths. C)For all x,ye V(G),there is a cycle through x and y. D)8(G)>1,and every pair of edges in G lies on a common cycle. 这里，最有意思的是如何推导出D。2-连通图的特征 这里，最有意思的是如何推导出D