Linear Algebra and Its Applications, Exercise 1.16.13

Exercise 1.6.13. Compute $A^TB$ , $B^TA$ , $AB^T$ , and $BA^T$ for the following matrices:

$A = \begin{bmatrix} 3 \\ 1 \end{bmatrix} \quad B = \begin{bmatrix} 2 \\ 2 \end{bmatrix}$

Answer: We have

$A^TB = \begin{bmatrix} 3&1 \end{bmatrix} \begin{bmatrix} 2 \\ 2 \end{bmatrix} = 8$

$B^TA = \begin{bmatrix} 2&2 \end{bmatrix} \begin{bmatrix} 3 \\ 1 \end{bmatrix} = 8$

$AB^T = \begin{bmatrix} 3 \\ 1 \end{bmatrix} \begin{bmatrix} 2&2 \end{bmatrix} = \begin{bmatrix} 6&6 \\ 2&2 \end{bmatrix}$

$BA^T = \begin{bmatrix} 2 \\ 2 \end{bmatrix} \begin{bmatrix} 3&1 \end{bmatrix} = \begin{bmatrix} 6&2 \\ 6&2 \end{bmatrix}$

Note that per Equation 1M(i) on page 47 we have

$(AB)^T = B^TA^T$

This means that

$(A^TB)^T = B^T(A^T)^T = B^TA$

and

$(AB^T)^T = (B^T)^TA^T = BA^T$

as confirmed by the computation above.

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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