围绕割点的重要概念之一：不可分割 A graph of order at least 3 is nonseparable iff every two vertices lie on a common cycle ■充分条件 口显然，删除任意一点，均不能使得任意两点不连通 。必要条件：反证法 口找到不成回路的最短uw路径P Why? 口d(u,v)=1,易证 口d(u,v)≥2，令k-是P中v的直接前驱节点 ■u,vk-1有回路C ■删去vk-1,u和v仍然相连：Q Uk=U 令x是C和Q的第一个交点 构造P',Q,可以形成ux+Q'+P'的uw回路
围绕割点的重要概念之一：不可分割 A graph of order at least 3 is nonseparable iff every two vertices lie on a common cycle ◼ 充分条件 ❑ 显然，删除任意一点，均不能使得任意两点不连通 ◼ 必要条件：反证法 ❑ 找到不成回路的最短𝑢𝑣路径𝑃 ❑ 𝑑 𝑢, 𝑣 = 1，易证 ❑ 𝑑 𝑢, 𝑣 ≥ 2,令𝑣𝑘−1是𝑃中𝑣的直接前驱节点 ◼ 𝑢, 𝑣𝑘−1有回路𝐶 ◼ 删去𝑣𝑘−1，𝑢和𝑣仍然相连: 𝑄 ◼ 令𝑥是𝐶和𝑄的第一个交点 ◼ 构造𝑃′ , 𝑄′ ,可以形成𝑢𝑥 + 𝑄′ + 𝑃′的𝑢𝑣回路 Why？
围绕割点的重要概念之二：Bock A maximal nonseparable subgraph of a graph Gis called a block of G. 问题3：极大还 是最大？
围绕割点的重要概念之二：Block A maximal nonseparable subgraph of a graph G is called a block of G. 问题3：极大还 是最大？
关于block的几个理解 A graph of order at least 3 is nonseparable iff every two vertices lie on a common cycle Uk-1 Uk=U 问题4：从这个定理及 证明中能否看出，不 可分离图在外表上具 有什么共性？
问题4：从这个定理及 证明中能否看出，不 可分离图在外表上具 有什么共性？ 关于block的几个理解 A graph of order at least 3 is nonseparable iff every two vertices lie on a common cycle
关于block的几个理解 Theorem 5.8 Let R be the relation defined on the edge set of a nontrivial connected graph G by e Rf,where e,f EE(G),if e f or e and f lie on a common cycle of G.Then R is an equivalence relation. 用等价关系来描述边之间的关系，到底有何用意？ 问题5：Block的“极大”特性和“等价类”有何关系？ Each subgraph of G induced by the edges in an equivalence class is in fact a block of G
关于block的几个理解 Theorem 5.8 Let 𝑅 be the relation defined on the edge set of a nontrivial connected graph G by 𝒆 𝑹 𝒇, where 𝑒, 𝑓 ∈ 𝐸(𝐺), if 𝑒 = 𝑓 or 𝑒 and 𝑓 lie on a common cycle of 𝐺. Then 𝑅 is an equivalence relation. 用等价关系来描述边之间的关系，到底有何用意？ Each subgraph of G induced by the edges in an equivalence class is in fact a block of G. 问题5：Block的“极大”特性和“等价类”有何关系？
间题6： 两个bock的“边界”是什么？ 1 U2 3 G:2 201 02 u3 01 V1 u W2 3 Bi B2 B3 Ba Figure 5.4:A connected graph and its blocks