Matrix Product (Conventional)/Examples
Examples of Matrix Product
$2 \times 2$ Real Matrices
Let $\mathbf A = \begin {pmatrix} p & q \\ r & s \end {pmatrix}$ and $\mathbf B = \begin {pmatrix} w & x \\ y & z \end {pmatrix}$ be order $2$ square matrices over the real numbers.
Then the matrix product of $\mathbf A$ with $\mathbf B$ is given by:
- $\mathbf A \mathbf B = \begin {pmatrix} p w + q y & p x + q z \\ r w + s y & r x + s z \end {pmatrix}$
$3 \times 3$ Matrix-Vector Multiplication Formula
The $3 \times 3$ matrix-vector multiplication formula is an instance of the matrix product operation:
- $\mathbf A \mathbf v = \begin{bmatrix}
a_{11} & a_{12} & a_{13} \\ a_{21} & a_{22} & a_{23} \\ a_{31} & a_{32} & a_{33} \\ \end{bmatrix} \begin{bmatrix} x \\ y \\ z \end{bmatrix} = \begin{bmatrix} a_{11} x + a_{12} y + a_{13} z \\ a_{21} x + a_{22} y + a_{23} z \\ a_{31} x + a_{32} y + a_{33} z \\ \end{bmatrix}$
Cayley's Motivation
Let there be $3$ Cartesian coordinate systems:
- $\tuple {x, y}$, $\tuple {x', y'}$, $\tuple {x, y}$
Let them be connected by:
- $\begin {cases} x' = x + y \\ y' = x - y \end {cases}$
and:
- $\begin {cases} x = -x' - y' \\ y = -x' + y' \end {cases}$
The relationship between $\tuple {x, y}$ and $\tuple {x, y}$ is given by:
- $\begin {cases}
x = -x' - y' = -\paren {x + y} - \paren {x - y} = -2 x \\ y = -x' + y' = -\paren {x + y} + \paren {x - y} = -2 y \end {cases}$
Arthur Cayley devised the compact notation that expressed the changes of coordinate systems by arranging the coefficients in an array:
- $\begin {pmatrix} 1 & 1 \\ 1 & -1 \end {pmatrix} \begin {pmatrix} -1 & -1 \\ -1 & 1 \end {pmatrix} = \begin {pmatrix} -2 & 0 \\ 0 & -2 \end {pmatrix}$
As such, he can be considered as having invented matrix multiplication.
Change of Axes
Consider the Cartesian coordinate system:
- $C := \tuple {x, y, z}$
Let $\mathbf A$ denote the square matrix:
- $\mathbf A = \begin {pmatrix} 0 & 1 & 0 \\ 1 & 0 & 0 \\ 0 & 0 & 1 \end {pmatrix}$
Then $\mathbf A$ has the effect of exchanging the $x$ and $y$ axes of $C$.
Column Matrix All $0$ except for one $1$
Let $\mathbf A$ be a matrix of order $m \times n$.
For $1 \le i \le n$, let $\mathbf e_i$ be the column matrix of order $n \times 1$ defined as:
- $e_k = \delta_{k i}$
where:
- $e_k$ is the element of $\mathbf e_i$ whose indices are $\tuple {k, 1}$
- $\delta_{k i}$ denotes the Kronecker delta.
Then $\mathbf A \mathbf e_i$ is the column matrix which is equal to the $i$th column of $\mathbf A$.
Arbitrary Matrices $1$
- $\begin {bmatrix} 2 & 1 & 0 \\ 3 & 0 & 7 \end {bmatrix} \begin {bmatrix} 2 & 3 & 5 & 8 \\ 4 & 8 & 6 & 1 \\ -1 & 7 & 0 & 7 \end {bmatrix} = \begin {bmatrix} 8 & 14 & 16 & 17 \\ -1 & 58 & 15 & 73 \end {bmatrix}$
Arbitrary Matrices $2$
- $\begin {bmatrix} 5 & 3 & 1 \end {bmatrix} \begin {bmatrix} 2 \\ 0 \\ 6 \end {bmatrix} = \begin {bmatrix} 16 \end {bmatrix}$
Arbitrary Matrices $3$
- $\begin {bmatrix} 2 \\ 0 \\ 6 \end {bmatrix} \begin {bmatrix} 5 & 3 & 1 \end {bmatrix} = \begin {bmatrix} 10 & 6 & 2 \\ 0 & 0 & 0 \\ 30 & 18 & 6 \end {bmatrix}$
Arbitrary Matrices $4$
- $\begin {bmatrix} 1 & 3 & -2 \\ -2 & -6 & 4 \\ 4 & 12 & -8 \end {bmatrix} \begin {bmatrix} 3 & -1 & 2 \\ 3 & 5 & -4 \\ 6 & 7 & -5 \end {bmatrix} = \begin {bmatrix} 0 & 0 & 0 \\ 0 & 0 & 0 \\ 0 & 0 & 0 \end {bmatrix}$