# Determine the Vertex Connectivity of a Graph

## Introduction

Vertex connectivity of a graph is an important concept in graph theory that helps us understand the resilience of a graph or network. In this blog post, we will be discussing what vertex connectivity is, its real-world applications, and how to determine the vertex connectivity of a graph using Python.

## What is Vertex Connectivity?

In a graph, the vertex connectivity is the minimum number of vertices that need to be removed to disconnect the graph. In other words, it is a measure of how strongly connected the nodes are in a graph. A higher vertex connectivity indicates a more resilient and robust network, as it takes more effort to disconnect the graph.

## Real-World Examples and Scenarios

Vertex connectivity has various real-world applications, such as:

- In communication networks, vertex connectivity can help determine the robustness of the network and ensure that data can be transmitted effectively even if some nodes fail.
- In social networks, vertex connectivity can be used to identify influential nodes or communities that play a crucial role in information dissemination.
- In transportation networks, vertex connectivity helps analyze the reliability of a transportation system by identifying critical nodes that, if removed, would significantly disrupt the system.

## Real-World Scenario and Technical Problem

Let's consider a transportation network in a city where intersections are represented as vertices and roads connecting them as edges. The city's transportation authority wants to assess the transportation network's robustness by determining the minimum number of intersections that need to be closed to disconnect the network.

## Problem Statement and Formal Definition

Given a connected, undirected graph G = (V, E), where V represents the set of vertices (intersections) and E represents the set of edges (roads), the objective is to find the vertex connectivity kappa(G) of the graph.

## Problem Statement and Real-World Scenario

The problem statement can be tied back to our real-world scenario by finding the minimum number of intersections (vertices) that need to be closed to disconnect the city's transportation network (graph).

## Solution to the Problem

To solve this problem, we will implement an algorithm based on Menger's theorem. Menger's theorem states that the minimum number of vertices needed to disconnect a graph is equal to the maximum number of vertex-independent paths between any two non-adjacent vertices. Vertex-independent paths are paths that do not share any vertices except their endpoints.

## Step-by-Step Solution with the Real-World Scenario

- Choose a pair of non-adjacent vertices u and v in the graph.
- Find all vertex-independent paths from u to v.
- Count the number of vertex-independent paths.
- Repeat steps 1-3 for all pairs of non-adjacent vertices and find the maximum number of vertex-independent paths.
- The vertex connectivity kappa(G) is equal to the maximum number of vertex-independent paths.

## Actual Code Solution with High-Level Comments

```
import networkx as nx
def find_vertex_connectivity(graph):
# Step 1: Get all pairs of non-adjacent vertices
non_adjacent_pairs = [(u, v) for u in graph for v in graph if u != v and not graph.has_edge(u, v)]
# Step 2: Find the maximum number of vertex-independent paths
max_vertex_independent_paths = 0
for u, v in non_adjacent_pairs:
# Step 3: Find all vertex-independent paths from u to v
vertex_independent_paths = nx.node_disjoint_paths(graph, u, v)
# Step 4: Count the number of vertex-independent paths
num_vertex_independent_paths = len(list(vertex_independent_paths))
# Step 5: Update the maximum number of vertex-independent paths
max_vertex_independent_paths = max(max_vertex_independent_paths, num_vertex_independent_paths)
# Step 6: Return the vertex connectivity kappa(G)
return max_vertex_independent_paths
# Create a graph representing the city's transportation network
G = nx.Graph()
G.add_edges_from([(1, 2), (1, 3), (2, 3), (2, 4), (2, 5), (3, 5), (4, 5)])
# Call the function with the transportation network graph
vertex_connectivity = find_vertex_connectivity(G)
print("Vertex connectivity:", vertex_connectivity)
```

## Explanation of the Code Solution

In this code solution, we use the NetworkX library to represent the graph and perform graph-related operations. We iterate through all pairs of non-adjacent vertices and use `nx.node_disjoint_paths()`

to find all vertex-independent paths between them. By counting the number of vertex-independent paths and keeping track of the maximum, we can determine the vertex connectivity.

## How the Solution Can Solve Other Similar Real-World Problems

The solution provided in this blog post can be applied to other real-world problems where vertex connectivity is essential, such as communication networks and social networks. By modifying the graph representation and input data, this solution can be used to analyze the robustness and resilience of various networks.