# Compute the Degree Distribution of a Graph

## Introduction to Degree Distribution of a Graph

In graph theory, the degree of a vertex is the number of edges connected to it. Degree distribution is a probability distribution of the degrees of vertices in a graph. It is an important measure to analyze various properties of a graph, such as its robustness and connectivity.

Degree distribution can be used in various real-world scenarios, such as analyzing social networks, understanding the structure of the internet, and studying biological networks.

## Real-world Example: Social Network Analysis

Consider the case of a social network, where users are represented as vertices and their connections (friendships) are represented as edges. Degree distribution can help us understand the structure of the network, such as the presence of highly connected individuals who act as hubs, or the existence of isolated communities.

In this context, we can create a technical problem related to degree distribution: finding the most influential users in the social network.

## Problem Statement

Given a graph G(V, E) representing a social network, where V is the set of vertices (users) and E is the set of edges (friendships), find the top k most influential users, where k is a positive integer. The influence of a user is defined as the degree of the vertex representing the user in the graph.

## Connecting the Problem Statement to the Real-world Scenario

The problem statement is directly connected to our real-world scenario of social network analysis. By finding the most influential users, we can identify the users who have the highest number of connections, and therefore, have a greater potential to spread information or influence others in the network.

## Solution to the Problem

To solve this problem, we can follow these steps:

- Calculate the degree of each vertex in the graph.
- Sort the vertices based on their degree in descending order.
- Select the top k vertices with the highest degree.

## Step-by-step Solution using Python

Let's implement the solution using Python.

### Step 1: Calculate the degree of each vertex

First, we need to calculate the degree of each vertex in the graph. We can represent the graph using an adjacency list and then calculate the degree by counting the number of neighbors for each vertex.

```
# Calculate the degree of each vertex
def calculate_degrees(graph):
degrees = {}
for vertex in graph:
degrees[vertex] = len(graph[vertex])
return degrees
```

### Step 2: Sort the vertices based on their degree

Next, we need to sort the vertices based on their degree in descending order.

```
# Sort the vertices based on their degree
def sort_vertices_by_degree(degrees):
return sorted(degrees.items(), key=lambda x: x[1], reverse=True)
```

### Step 3: Select the top k vertices with the highest degree

Finally, we can select the top k vertices with the highest degree.

```
# Find the top k most influential users
def find_top_k_influential_users(sorted_degrees, k):
return [vertex for vertex, degree in sorted_degrees[:k]]
```

## Code Solution

Here's the complete code solution with high-level comments.

```
# Calculate the degree of each vertex
def calculate_degrees(graph):
degrees = {}
for vertex in graph:
degrees[vertex] = len(graph[vertex])
return degrees
# Sort the vertices based on their degree
def sort_vertices_by_degree(degrees):
return sorted(degrees.items(), key=lambda x: x[1], reverse=True)
# Find the top k most influential users
def find_top_k_influential_users(sorted_degrees, k):
return [vertex for vertex, degree in sorted_degrees[:k]]
# Example graph representing a social network
graph = {
'A': ['B', 'C', 'D'],
'B': ['A', 'D'],
'C': ['A', 'D', 'E'],
'D': ['A', 'B', 'C', 'E'],
'E': ['C', 'D']
}
# Calculate the degrees and find the top 2 most influential users
degrees = calculate_degrees(graph)
sorted_degrees = sort_vertices_by_degree(degrees)
top_users = find_top_k_influential_users(sorted_degrees, 2)
print(top_users) # Output: ['D', 'A']
```

## Explanation of the Code Solution

In the code solution, we first calculate the degree of each vertex in the graph using the `calculate_degrees`

function. We then sort the vertices based on their degree using the `sort_vertices_by_degree`

function. Finally, we find the top k most influential users using the `find_top_k_influential_users`

function.

The solution is intuitive and easy to understand, as we simply count the number of connections for each user, sort them based on the count, and select the top k users.

## Solving Similar Real-world Problems

The same approach can be applied to other real-world problems involving degree distribution, such as finding the most connected websites in the internet or identifying the most important proteins in a biological network. By understanding the concept of degree distribution, we can analyze various properties of networks and find solutions to complex problems in different domains.