## Linear Algebra and Its Applications, Exercise 2.6.18

Exercise 2.6.18. Given a vector $\begin{bmatrix} x_1&x_2&x_3 \end{bmatrix}^T$ in $\mathbb{R}^3$ find a matrix $A$ that produces a corresponding vector $\begin{bmatrix} 0&x_1&x_2&x_3 \end{bmatrix}^T$ in $\mathbb{R}^4$ in which all entries are shifted right one place. Find a second matrix $B$ that takes a  vector $\begin{bmatrix} x_1&x_2&x_3&x_4 \end{bmatrix}^T$ in $\mathbb{R}^4$ and produces the vector $\begin{bmatrix} x_2&x_3&x_4 \end{bmatrix}^T$ in $\mathbb{R}^3$ in which all entries are shifted left one place. What are the product matrices $AB$ and $BA$ and what effects do they have?

Answer: We can construct $A$ by considering its effect on the elementary vectors in $\mathbb{R}^3$

$e_1 = \begin{bmatrix} 1 \\ 0 \\ 0 \end{bmatrix} \qquad e_2 = \begin{bmatrix} 0 \\ 1 \\ 0 \end{bmatrix} \qquad e_3 = \begin{bmatrix} 0 \\ 0 \\ 1 \end{bmatrix}$

Applying $A$ to each of $e_1$ through $e_3$ respectively will shift each of their entries to the right (or down, depending on your point of view) and produce the following vectors in $\mathbb{R}^4$:

$\begin{bmatrix} 0 \\ 1 \\ 0 \\ 0 \end{bmatrix} \qquad \begin{bmatrix} 0 \\ 0 \\ 1 \\ 0 \end{bmatrix} \qquad \begin{bmatrix} 0 \\ 0 \\ 0 \\ 1 \end{bmatrix}$

As discussed in the answer to exercise 2.6.16, we can do this by having each column of $A$ be the vector into which the corresponding elementary vector should be transformed. We thus have

$A = \begin{bmatrix} 0&0&0 \\ 1&0&0 \\ 0&1&0 \\ 0&0&1 \end{bmatrix}$

so that

$Ax = \begin{bmatrix} 0&0&0 \\ 1&0&0 \\ 0&1&0 \\ 0&0&1 \end{bmatrix} \begin{bmatrix} x_1 \\ x_2 \\ x_3 \end{bmatrix} = \begin{bmatrix} 0 \\ x_1 \\ x_2 \\ x_3 \end{bmatrix}$

Now consider $B$. We can construct $B$ by considering its effect on the elementary vectors in $\mathbb{R}^4$

$e_1 = \begin{bmatrix} 1 \\ 0 \\ 0 \\ 0 \end{bmatrix} \qquad e_2 = \begin{bmatrix} 0 \\ 1 \\ 0 \\ 0 \end{bmatrix} \qquad e_3 = \begin{bmatrix} 0 \\ 0 \\ 1 \\ 0 \end{bmatrix} \qquad e_4 = \begin{bmatrix} 0 \\ 0 \\ 0 \\ 1 \end{bmatrix}$

Applying $B$ to each of $e_1$ through $e_4$ respectively will shift each of their entries to the left (or up) and produce the following vectors in $\mathbb{R}^3$:

$\begin{bmatrix} 0 \\ 0 \\ 0 \end{bmatrix} \qquad \begin{bmatrix} 1 \\ 0 \\ 0 \end{bmatrix} \qquad \begin{bmatrix} 0 \\ 1 \\ 0 \end{bmatrix} \qquad \begin{bmatrix} 0 \\ 0 \\ 1 \end{bmatrix}$

We thus have

$B = \begin{bmatrix} 0&1&0&0 \\ 0&0&1&0 \\ 0&0&0&1 \end{bmatrix}$

so that

$Bx = \begin{bmatrix} 0&1&0&0 \\ 0&0&1&0 \\ 0&0&0&1 \end{bmatrix} \begin{bmatrix} x_1 \\ x_2 \\ x_3 \\ x_4 \end{bmatrix} = \begin{bmatrix} x_2 \\ x_3 \\ x_4 \end{bmatrix}$

Constructing the product matrices we have

$AB = \begin{bmatrix} 0&0&0 \\ 1&0&0 \\ 0&1&0 \\ 0&0&1 \end{bmatrix} \begin{bmatrix} 0&1&0&0 \\ 0&0&1&0 \\ 0&0&0&1 \end{bmatrix} = \begin{bmatrix} 0&0&0&0 \\ 0&1&0&0 \\ 0&0&1&0 \\ 0&0&0&1 \end{bmatrix}$

and

$BA = \begin{bmatrix} 0&1&0&0 \\ 0&0&1&0 \\ 0&0&0&1 \end{bmatrix} \begin{bmatrix} 0&0&0 \\ 1&0&0 \\ 0&1&0 \\ 0&0&1 \end{bmatrix} = \begin{bmatrix} 1&0&0 \\ 0&1&0 \\ 0&0&1 \end{bmatrix}= I$

The product $matrix BA$ corresponds to taking a vector $\begin{bmatrix} x_1&x_2&x_3 \end{bmatrix}^T$ in $\mathbb{R}^3$ and shifting the entries right to produce a corresponding vector $\begin{bmatrix} 0&x_1&x_2&x_3 \end{bmatrix}^T$ in $\mathbb{R}^4$ and then shifting the entries left to produce the original vector $\begin{bmatrix} x_1&x_2&x_3 \end{bmatrix}^T$ in $\mathbb{R}^3$. So $BA$ preserves all vectors and is thus equal to the identity matrix on $\mathbb{R}^3$.

On the other hand the product $matrix AB$ corresponds to taking a vector $\begin{bmatrix} x_1&x_2&x_3&x_4 \end{bmatrix}^T$ in $\mathbb{R}^4$ and shifting the entries left to produce a corresponding vector $\begin{bmatrix} x_2&x_3&x_4 \end{bmatrix}^T$ in $\mathbb{R}^3$ and then shifting the entries right to produce the vector $\begin{bmatrix} 0&x_2&x_3&x_4 \end{bmatrix}^T$ in $\mathbb{R}^4$. So the net effect of $AB$ is to change the first entry of a vector in $\mathbb{R}^4$ to zero and preserve the second, third, and fourth entries.

NOTE: This continues a series of posts containing worked out exercises from the (out of print) book Linear Algebra and Its Applications, Third Edition by Gilbert Strang.

If you find these posts useful I encourage you to also check out the more current Linear Algebra and Its Applications, Fourth Edition, Dr Strang’s introductory textbook Introduction to Linear Algebra, Fourth Edition and the accompanying free online course, and Dr Strang’s other books.

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