Networks & Matrices

Introduction to Networks & Matrices

What is a network?

A network is a weighted graph
- A network is used to model real-world situations
- The weight of an edge usually represents a measure such as distance or time

What is a matrix?

A matrix can be used to represent a graph or network in the form of a table
- The elements within the matrix give information about the connections between the different vertices
A matrix is typically set up with the following labelling system
- Rows are labelled with the 'from' vertices
- Columns are labelled with the 'to' vertices

What is an adjacency matrix?

An adjacency matrix is a square matrix
- All of the vertices in the graph are listed as the headings for both the rows and columns
- The leading diagonal is the the line going from the top left cell to the bottom right cell
An adjacency matrix can be used to show the number of direct connections between two vertices
An entry of 1 in the matrix means that there is a direct connection (i.e. an edge) between that pair of vertices
An entry of 0 in the matrix means that there is no direct connection between that pair of vertices
In a simple graph the only entries are 0 or 1
A loop is indicated in an adjacency matrix with a value in the leading diagonal
- In an undirected matrix the value in the leading diagonal will be 2
  - You can use the loop to travel out of and into the vertex in two different directions
- In a directed matrix, if the loop has direction, the value in the leading diagonal will be 1
  - You can only travel along the loop out of and back into the vertex in one direction
  - An undirected loop in a directed matrix will still have a value of 2
- For a graph with no loops every entry in the leading diagonal will be 0
An undirected graph is symmetrical about the leading diagonal
- A directed graph is not symmetrical about the leading diagonal

What is a distance matrix?

A distance matrix is different to an adjacency matrix
- The value in each cell is the weight of the edge connecting that pair of vertices
- Weight could be cost, distance, time etc.
An empty cell is used to indicate that there is no connection (i.e. no edge) between a pair of vertices
An undirected network is symmetrical about the leading diagonal
- A directed network is not symmetrical about the leading diagonal
When drawing a network from its distance matrix be careful when labelling the edges
- For an undirected graph the two cells between a pair of vertices will be the same
  - Connect the vertices with one edge labelled with the relevant weight
- For a directed graph where two cells between a pair of vertices have different values
  - Draw two edges between the vertices
  - Label each with the correct weight and direction
A distance matrix can be used to work out the weight of different walks in the network

Worked example

Write down the adjacency matrix for the graph shown below.

3-10-2-ib-hl-ai-walks--adjacency-matrices-we-1

$\begin{matrix} \begin{matrix} A & B & C & D & E \end{matrix} \\ \begin{matrix} A \\ B \\ C \\ D \\ E \end{matrix} & (\begin{matrix} 0 & 1 & 1 & 0 & 1 \\ 1 & 0 & 0 & 0 & 0 \\ 1 & 0 & 1 & 1 & 1 \\ 0 & 0 & 0 & 0 & 1 \\ 2 & 1 & 1 & 1 & 2 \end{matrix}) \end{matrix}$

The table below shows the time taken in minutes to travel by car between 4 different towns.

	A	B	C	D
A		16	35
B	16		20	18
C	35	20		34
D		23	34

Draw the network described by this distance matrix.

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Networks & Matrices (Edexcel International A Level Maths: Decision 1)

Revision Note

Introduction to Networks & Matrices

What is a network?

What is a matrix?