The Matrix Representation of a Relation

# The Matrix Representation of a Relation

Recall from the Hasse Diagrams page that if $X$ is a finite set and $R$ is a relation on $X$ then we can construct a Hasse Diagram in order to describe the relation $R$. We will now look at another method to represent relations with matrices.

 Definition: Let $X = \{ x_1, x_2, ..., x_n \}$ be a finite $n$-element set and let $R$ be a relation on $X$. The Matrix Representation of $R$ on $X$ is defined to be the $n \times n$ matrix $M = (m_{ij} )$ where the entires for $i, j \in \{1, 2, ..., n \}$ are given by $m_{ij} = \left\{\begin{matrix} 1 & \mathrm{if} \: x_i \: R \: x_j \\ 0 & \mathrm{if} \: x_i \: \not R \: x_j \end{matrix}\right.$.

For example, consider the set $X = \{1, 2, 3 \}$ and let $R$ be the relation where for $x, y \in X$ we have that $x \: R \: y$ if $x + y$ is divisible by $2$, that is $(x + y) \equiv 0 \pmod 2$. If we let $x_1 = 1$, $x_2 = 2$, and $x_3 = 3$ then we see that the following ordered pairs are contained in $R$:

(1)
\begin{align} \quad R = \{ (x_1, x_1), (x_1, x_3), (x_2, x_3), (x_3, x_1), (x_3, x_3) \} \subset X \times X \end{align}

Let $M$ be the matrix representation of $R$. Then $m_{11}, m_{13}, m_{22}, m_{31}, m_{33} = 1$ and $m_{12}, m_{21}, m_{23}, m_{32} = 0$ and:

(2)
\begin{align} \quad M = \begin{bmatrix} 1 & 0 & 1\\ 0 & 1 & 0\\ 1 & 0 & 1 \end{bmatrix} \end{align}

If $X$ is a finite $n$-element set and $\emptyset$ is the empty relation on $X$ then the matrix representation of $\emptyset$ on $X$ which we denote by $M_{\emptyset}$ is equal to the $n \times n$ zero matrix because for all $x_i, x_j \in X$ where $i, j \in \{1, 2, ..., n \}$ we have by definition of the empty relation that $x_i \: \not R \: x_j$ so $m_{ij} = 0$ for all $i, j$:

(3)
\begin{align} \quad M_{\emptyset} = \begin{bmatrix} 0 & 0 & \cdots & 0\\ 0 & 0 & \cdots & 0\\ \vdots & \vdots & \ddots & \vdots\\ 0 & 0 & \cdots & 0 \end{bmatrix} \end{align}

On the other hand if $X$ is a finite $n$-element set and $\mathcal U$ is the universal relation on $X$ then the matrix representation of $\mathcal U$ on $X$ which we denote by $M_{\mathcal U}$ is equal to the $n \times n$ matrix whoses entries are all $1$'s because for all $x_i, x_j \in X$ where $i, j \in \{ 1, 2, ..., n \}$ we have by definition of the universal relation that $x_i \: R \: x_j$ so $m_{ij} = 1$ for all $i, j$:

(4)
\begin{align} \quad M_{\mathcal U} = \begin{bmatrix} 1 & 1 & \cdots & 1\\ 1 & 1 & \cdots & 1\\ \vdots & \vdots & \ddots & \vdots\\ 1 & 1 & \cdots & 1 \end{bmatrix} \end{align}