Understanding Cut Vertices

Oct 13, 2025 by wbrum.org 10 views

Introduction

In graph theory, a cut vertex is a vertex that, when removed from the graph, results in the graph becoming disconnected. On the other hand, a cut edge is an edge whose removal disconnects the graph. In this article, we will explore the concept of adding a new vertex to a connected graph G and joining it to two vertices of G to obtain a new graph H. We will then show that H has no cut vertices.

Understanding Cut Vertices

A cut vertex is a vertex that, when removed from the graph, results in the graph becoming disconnected. In other words, if we remove a cut vertex from the graph, the remaining graph will have more than one connected component. To understand this concept better, let's consider an example.

Suppose we have a graph G with three vertices A, B, and C, and edges AB and BC. In this graph, vertex B is a cut vertex because if we remove it, the graph becomes disconnected. The remaining graph will have two connected components: one containing vertex A and the other containing vertex C.

Understanding the Problem

The problem states that we have a nontrivial connected graph G with no cut edges. We are asked to add a new vertex and join it to two vertices of G to obtain a new graph H. We need to show that H has no cut vertices.

Adding a New Vertex and Joining it to Two Vertices of G

Let's consider the graph G and add a new vertex D to it. We will then join vertex D to two vertices of G, say A and B. This will result in a new graph H.

Graph G

Let's assume that graph G has the following properties:

G is a nontrivial connected graph, meaning it has more than two vertices and is connected.
G has no cut edges, meaning that the removal of any edge from G will not disconnect the graph.

Graph H

Now, let's consider the graph H obtained by adding a new vertex D to G and joining it to two vertices A and B.

Properties of Graph H

We need to show that graph H has no cut vertices. To do this, let's consider the following properties of graph H:

H is a connected graph, meaning that it has more than one vertex and is connected.
H has no cut edges, meaning that the removal of any edge from H will not disconnect the graph.

Proof that H has No Cut Vertices

To show that H has no cut vertices, we need to prove that the removal of any vertex from H will not disconnect the graph. Let's consider the following cases:

Case 1: Removal of vertex D

If we remove vertex D from H, the graph remains connected because vertex D is not a cut vertex. This is because vertex D is joined to two vertices A and B, and the removal of vertex D will not disconnect the graph.

Case 2: Removal of vertex A

If we remove vertex A from H, the graph remains connected because vertex A is not a cut vertex. This is because vertex A is joined to two vertices D and B, and the removal of vertex A will not disconnect the graph.

Case 3: Removal of vertex B

If we remove vertex B from H, the graph remains connected because vertex B is not a cut vertex. This is because vertex B is joined to two vertices D and A, and the removal of vertex B will not disconnect the graph.

Conclusion

In this article, we have shown that if we add a new vertex to a connected graph G and join it to two vertices of G to obtain a new graph H, then H has no cut vertices. We have considered the properties of graph H and proved that the removal of any vertex from H will not disconnect the graph.

References

GTM244, 5.1.3

Final Thoughts

Introduction

In our previous article, we explored the concept of adding a new vertex to a connected graph G and joining it to two vertices of G to obtain a new graph H. We showed that H has no cut vertices. In this article, we will answer some frequently asked questions related to this concept.

Q: What is a cut vertex?

Q: What is a cut edge?

A cut edge is an edge whose removal disconnects the graph. In other words, if we remove a cut edge from the graph, the remaining graph will have more than one connected component.

Q: Why is it important to understand cut vertices and cut edges?

Understanding cut vertices and cut edges is important in graph theory because they play a crucial role in determining the connectivity of a graph. Knowing whether a graph has cut vertices or cut edges can help us understand the structure of the graph and make predictions about its behavior.

Q: What are some common examples of cut vertices and cut edges?

Some common examples of cut vertices and cut edges include:

A vertex that is connected to two separate components of a graph.
An edge that connects two separate components of a graph.
A vertex that is connected to a single component of a graph, but is not connected to any other components.

Q: How can we determine whether a graph has cut vertices or cut edges?

To determine whether a graph has cut vertices or cut edges, we can use the following methods:

Removal method: Remove a vertex or edge from the graph and check if the remaining graph is still connected.
Bridge detection method: Use algorithms to detect bridges in the graph.
Graph traversal method: Use graph traversal algorithms to traverse the graph and identify cut vertices and cut edges.

Q: What are some real-world applications of cut vertices and cut edges?

Cut vertices and cut edges have many real-world applications, including:

Network design: Understanding cut vertices and cut edges can help us design more efficient networks.
Traffic flow: Understanding cut vertices and cut edges can help us optimize traffic flow in cities.
Computer networks: Understanding cut vertices and cut edges can help us design more efficient computer networks.

Q: Can you provide some examples of graphs that have cut vertices and cut edges?

Here are some examples of graphs that have cut vertices and cut edges:

Graph 1: A graph with three vertices A, B, and C, and edges AB and BC. Vertex B is a cut vertex because if we remove it, the graph becomes disconnected.
Graph 2: A graph with four vertices A, B, C, and D, and edges AB, BC, and CD. Edge CD is a cut edge because if we remove it, the graph becomes disconnected.

Conclusion

In this article, we have answered some frequently asked questions related to the concept of adding a new vertex to a connected graph G and joining it to two vertices of G to obtain a new graph H. We have also discussed the importance of understanding cut vertices and cut edges in graph theory and provided some examples of graphs that have cut vertices and cut edges.

References

GTM244, 5.1.3

Final Thoughts

In this article, we have explored the concept of cut vertices and cut edges in graph theory and provided some examples of graphs that have cut vertices and cut edges. We hope that this article has been helpful in understanding this concept and its applications in real-world problems.