# Chapter 5: Connectivity Section 5.1: Vertex- and Edge-Connectivity

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1 Chapter 5: Connectivity Section 5.1: Vertex- and Edge-Connectivity Let G be a connected graph. We want to measure how connected G is. Vertex cut: V 0 V such that G V 0 is not connected Edge cut: E 0 E such that G E 0 is not connected Vertex connectivity of connected graph: κ v (G) = minimum size V 0 such that G V 0 is disconnected or a single vertex. (We will say disconnected to mean either). κ v (K n ) = n 1. If κ v (G) k then you must delete at least k vertices to disconnect G. G is k-vertexconnected then it has this property. Another way to say this: result is connected when you delete any k 1 vertices. Edge connectivity of connected graph: κ e (G) = minimum size E 0 such that G E 0 is disconnected. If κ e (G) k then you must delete at least k edges to disconnect G. Result is connected when you delete any k 1 edges. G is k-edge-connected then it has this property. κ e (G) δ. Reason: you can disconnect graph by deleting δ edges. So the minimum number is δ. Partition cut: an edge cut with the following property: you can 2-color the edges in the cut. Proposition 5.1.2: A graph G is k-connected if and only if every partition-cut contains at least k edges. Proof: Assume G is k-connected. Then no edge cut contains k 1 edges, in particular any partition-cut. So every partition-cut contains at least k edges. Conversely, suppose every partition-cut contains at at least k edges. Suppose that G E 0 is not connected. Then there is a subset E 1 E 0 in which G E 1 has exactly two components. Color the vertices in one of the components 0, and color the vertices in the other components 1. Let P be the edges in G which have edges with opposite color endpoints. Claim: P E 1. Reason: If there is an edge in P but not in E 1, it would connect the two components of G E 1, which is impossible. Therefore E 0 E 1 P k. Hence G is k-connected. Theorem: κ e (G) κ v (G). Proof: By induction on κ e (G). If κ e (G) = 1, then G has a bridge edge. If there are exactly 2 vertices, then deleting a vertex reduces graph to single vertex. If 3 vertices, then deleting the right vertex will disconnect graph. Hence κ v (G) = 1. Now consider κ e (G) = k. Pick any edge e in a cut of size k. Then κ e (G e) k 1. Hence κ v (G e) k 1. Therefore by the induction hypothesis it is possible to disconnect G e by removing k 1 vertices. If in removing V k 1 we also removed e, then removing V k 1

2 disconnects G. If we did not remove e, then e is a bridge edge of G V k 1. If just two vertices in G V k 1, then removing another vertex takes to one vertex, which disconnects G. If more than two vertices, use the same argument as above to remove another vertex. Hence we can disconnect G with k vertices. Hence κ v (G) k = κ e (G). Definition: two paths from u to v (u v) are internally disjoint if they have no common internal vertex. They can be glued together to form a cycle. Theorem: Let G be connected with 3 or more vertices. G is 2-connected iff for every pair u v there are two internally disjoint paths between them. Proof: Assume the condition on paths holds. We must show that no vertex disconnects graph. Cut x, and consider u v in G x. There are two internally disjoint paths from u to v in G. x cannot be internal to both. Therefore one of them survives in G x, and this supplies a path between them in G x. Conversely, assume G is 2-connected, and let u v be given. Suppose every pair of distinct paths between then share an internal vertex somewhere. We will construct internally disjoint paths by induction on the distance between u and v. distance = 1: show they belong to a cycle. κ e (G) κ(g) = 2, hence cutting one edge doesn t disconnect graph. Cut uv edge. There is still path from u to v. This is internally disjoint to the uv edge. Now assume this can be done for distance < k. Consider d(u, v) = k. Let d(u, w) = k 1, where w and v are connected by an edge. There are internally disjoint paths between u and w by induction hypothesis. Call them P and Q. Let R be uv path in G w. Case 1: R never intersects P or Q. Use R and P + wv. Case 2: R intersects P or Q. Let z be the last vertex it hits, without loss of generality in P. First path: P from u to z then R from z to v. Second path: Q from u to w then wz. NOTE: typo in problem Expansion Lemma: If G is k-connected, and we add a new vertex V with edges to k existing vertices and call this G, then G is also k-connected. Proof: Cut k 1 vertices. We must show that G V k 1 is still connected. We know that G V k 1 is still connected. If V was deleted, then G V k 1 = G V k 1. But if V was not deleted, there is surviving edge from V to G, so there are paths from V to every other vertex in G V k 1. Theorem: Let G be graph with at least 3 vertices. FAE: 1. G is connected and has no cut-vertex. 2. G is 2-connected. 3. Every pair u v has 2 internally disjoint paths. 4. Every pair u v belongs to cycle.

3 5. There are no isolated vertices, and every pair of edges belongs to a cycle. Proof: 1 through 4 are equivalent. 4 implies 5: Assume every pair of vertices belongs to a cycle. This implies 1, so we can assume we have all the properties of 1 through 4. Clearly there are no isolated vertices. Let uv and xy be two edges. If they share and endpoint, we can delete the common endpoint and there will still be a path between the two other endpoints. Hence these edges belong to a cycle. Now suppose uv and xy do not share an endpoint. Create new vertices W and Z and add them to G, connecteding W to u, v and Z to x, y. By expansion lemma, G is 2-connected. Therefore W and Z belong to cycle of G. This can be contracted to cycle through the two edges. 5 implies 1: First show connected. Let u and v be given. If uv edge, they are connected by path. If no uv edge, they belong to two edges which belong to cycle, hence they are connected by path. Next show no cut-vertex. Let x be vertex in graph. Cut x. We must show G x is connected. Let a, b be two vertices left over. If no edge, they belong to a cycle in G by the argument above, hence to two internally disjoint paths in G. One of these survives in G x. Section 5.2: Constructing Reliable Networks Lemma 5.2.1: If you add a path to a 2-connected graph you get a 2-connected graph. Proof: It is still true that every pair of vertices lies on a cycle. Theorem 5.2.2: G is 2-connected if and only if it can be obtained by adding paths to a cycle. Proof: Sufficiency is clear by the lemma. Now assume G is 2-connected. We will construct G by adding paths to a cycle as follows: Since G is 2-connected, it contains a cycle. Therefore it has a subgraph which can be generated by an ear decomposition. We will now show that any proper ear-decomposition subgraph H can be enlarged to a larger one H. Eventually we arrive at G. Let uv be an edge in G which does not belong to H. If both u and v are vertices in H, then the new ear-decomposition subgraph is H + uv and we are done. Otherwise, let xy be any edge in H. Then uv and xy belong to a cycle C of G since G is 2-connected. Since either u or v does not belong to H, we can use C to construct a new ear for H. Lemma 5.2.3: If you add a path or a cycle to a 2-edge-connected graph you get a 2-edge-connected graph. Proof: If no bridges to begin with, then no bridges after the addition.

4 Theorem 5.2.2: G is 2-edge-connected if and only if it can be obtained by adding paths or cycles to a cycle. Proof: Sufficiency is clear by the lemma. Conversely, let G be any 2-edge-connected graph. G contains cycle. We will show that we can continue to extend it by paths and cycles until we get to G. Let H be subgraph of G which results from cycle by adding paths and cycles. Suppose H contains all vertices of G. Then any uv edge not in H can be added to H, creating larger thingy. If on the other hand G contains vertex not in H, then by connectedness there is a frontier edge from vertex in H to vertex not in H. It cannot be a bridge edge of G, so it lives in cycle of G. Follow the cycle both directions until it hooks up with H. If it hooks up with only one vertex of H, it is a closed ear. Otherwise it is an open ear. Add to H, producing H. See Theorem for characterizations of 3-connected graphs. See Theorem for a characterization of k-connected graphs. Proposition 5.2.6: A k-connected graph on n vertices has at least kn 2 Proof: We have 2e nδ nk by the vertex-degree-sum theorem. Specialization: A 2r-connected graph on n vertices at at least rn edges. Achieving this bound: H 2r,n is 2r-connected and has rn edges (2r < n). Construction: vertices are [0], [1],..., [n 1] modulo n. edges. Neighbors of the vertex [a] are [a + 1] through [a + r] and [a 1] through [a r]. These are distinct vertices: If [a p] = [a + q] then p + q is divisible by n. But p + q 2r < n, hence p = q = 0. Contradiction. So the degree of [a] is 2r. This implies that H 2r,n has rn edges. Removing 2r vertices it is always possible to isolate a vertex and disconnect the graph. This makes κ v (H 2r,n ) 2r. Now we must show that if we remove 2r 1 vertices the resulting graph is connected. Remove 2r 1 vertices. Let [u] and [v] remain. If they are neighbors, we re done. Otherwise, the circular path between them in either direction involves r or more internal vertices. Now in one of these directions, at least one internal vertex [z] remains after cutting the 2r 1 vertices. So can we can take an edge from [u] to [z]. If the gap from [z] to [v] is still has r or more internal vertices, we can take an edge from [z] to an internal vertex [z ] in the same direction. Keep on going, then eventually take an edge to [v]. Section 5.3: Menger s Theorems u, v separating set S: No path from u to v in G S. S could be set of vertices or set of edges. Theorem (Menger s Theorem): Let u v be such that uv is not an edge in a connected graph G. Then the maximum number of internally disjoint uv paths in G is equal to the minimum number of vertices in a u, v separating set.

5 Proof: We ll do the easy part now, and use Network Flows in Chapter 13 to do the hard part. We will just show that if P is a set of internally disjoint uv paths and S is any set of vertices whose removal separates u and v, then P S. Let p be one of the paths. At least one internal vertex of p must belong to S, otherwise S doesn t separate. So every p P contributes a vertex to S, and since internally disjoint they must be distinct vertices. Hence P S. Corollary: whenever P = S we must have P max and S min. Definition: κ v (s, t) = minimum number of vertices needed to separate s from t, where s t are not adjacent. Lemma 5.3.5: κ v (G) = min κ v (s, t). Proof: Let κ v (s, t) be minimum possible. If you can separate the graph with κ v (s, t) 1, then you can separate a pair of vertices with this many contradiction. Therefore κ v (G) κ v (s, t). On the other hand, removing κ v (s, t) disconnects s from t, hence disconnects the graph, hence κ v (G) κ v (s, t). So they are equal. Theorem 5.3.6: G is k-connected if and only if for each pair of vertices s t there are at least k internally disjoint st paths in G. Proof: Assume G is k-connected. Let s, t be given, and suppose there are at most k 1 internally disjoint st paths in G. Since the maximum number of internally disjoint paths is < k, by Menger s Theorem we can say that the minimum number of vertices needed to separate s from t is < k. This contradicts k-connected. Hence there at least k internally disjoint st paths in G. Conversely, suppose there are at least k internally disjoint st paths per s t in G. Suppose it is possible to separate G by removing k 1 vertices. Then it is possible to separate a pair s, t by removing k 1 vertices. But this leaves one of the paths intact contradiction. Therefore G cannot be separated by removing k 1 vertices, and G is k-connected. Corollary 5.3.7: Let G be a k-connected graph and let v 0 through v k be distinct vertices in G. Then there are internally disjoint paths from v 0 to v i for 1 i k. Proof: By the Expansion Lemma proved above, we can adjoin a new vertex V to G and edges from V to v 1 through v k, obtaining a k-connected graph G. There are at least k internally disjoint paths from v 0 to V. This implies that there are exactly k internally disjoint paths of the type desired. Theorem 5.3.8: Let G be k connected, where k 3. Then any set of k vertices in G lives in a cycle of G. Proof: We will grow a cycle, using what we know about 2-connected graphs. Let v 1 through v k be any collection of k vertices. through the first j of them by induction on j 2. We will prove that there is a cycle j = 2: There is a cycle called C 2 through v 1 and v 2 since G is 2-connected.

6 Assume there is a cycle C j through v 1 through v j, where j < k. If this cycle includes v j+1 there is nothing to prove. If not, there are internally disjoint paths from v j+1 through v 1 through v j. So there are internally disjoint paths from v j+1 to v 1 and v 2, and these don t contain any other vertex on C j. So we can build C j+1 which incorporates v j+1. See Figure 5.3.4, page 235.

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