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Linear Algebra : Symmetric Matrices

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Example Questions

Example Question #91 : Operations And Properties

$A= \begin{bmatrix} 0 & 8+7 i \\ -8+7i &0 \end{bmatrix}$

True or false: is a skew-Hermitian matrix.

Possible Answers:

False

True

Correct answer:

True

Explanation:

is a skew-Hermitian matrix if it is equal to the additive inverse of its conjugate transpose - that is, if

$A = -A^{*}$

To determine whether this is the case, first, find the transpose of by exchanging rows with columns in :

$A= \begin{bmatrix} 0 & 8+7 i \\ -8+7i &0 \end{bmatrix}$

$A^{T}= \begin{bmatrix} 0 & -8+7 i \\ 8+7i &0 \end{bmatrix}$

Obtain the conjugate transpose by changing each element to its complex conjugate:

$A^{*}= \begin{bmatrix} 0 & -8-7 i \\ 8-7i &0 \end{bmatrix}$

Now find the additive inverse of this by changing each entry to its additive inverse:

$-A^{*}= \begin{bmatrix} 0 & 8+7 i \\- 8+7i &0 \end{bmatrix} = A$

$A = -A^{*}$ , so is skew-Hermitian.

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Example Question #31 : Symmetric Matrices

$A= \begin{bmatrix} 1 & 8+7 i \\ -8+7i &1 \end{bmatrix}$

True or false: is a skew-Hermitian matrix.

Possible Answers:

True

False

Correct answer:

False

Explanation:

is a skew-Hermitian matrix if it is equal to the additive inverse of its conjugate transpose - that is, if

$A = -A^{*}$

To determine whether this is the case, first, find the transpose of by exchanging rows with columns in :

$A= \begin{bmatrix} 1 & 8+7 i \\ -8+7i &1 \end{bmatrix}$

$A^{T}= \begin{bmatrix}1 & -8+7 i \\ 8+7i &1 \end{bmatrix}$

Obtain the conjugate transpose by changing each element to its complex conjugate:

$A^{*}= \begin{bmatrix} 1 & -8-7 i \\ 8-7i &1\end{bmatrix}$

Now find the additive inverse of this by changing each entry to its additive inverse:

$-A^{*}= \begin{bmatrix} -1 & 8+7 i \\- 8+7i &-1 \end{bmatrix} \ne A$

$A \ne -A^{*}$ , so is not skew-Hermitian.

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Example Question #32 : Symmetric Matrices

$A= \begin{bmatrix} 1 & 3+ i & -2 - i \\ -3+i &1 & 5i \\ 2-i &5i & 1\end{bmatrix}$

True or false: is a skew-Hermitian matrix.

Possible Answers:

True

False

Correct answer:

False

Explanation:

is a skew-Hermitian matrix if it is equal to the additive inverse of its conjugate transpose - that is, if

$A = -A^{*}$

To determine whether this is the case, first, find the transpose of by exchanging rows with columns in :

$A= \begin{bmatrix} 1 & 3+ i & -2 - i \\ -3+i &1 & 5i \\ 2-i &5i & 1\end{bmatrix}$

$A^{T}= \begin{bmatrix} 1 & -3+i & 2-i \\ 3+ i &1 & 5i \\ -2 - i &5i & 1\end{bmatrix}$

Obtain the conjugate transpose by changing each element to its complex conjugate:

$A^{*}= \begin{bmatrix} 1 &- 3-i & 2+i \\ 3- i &1 & -5i \\ -2 + i &-5i & 1\end{bmatrix}$

Now find the additive inverse of this by changing each entry to its additive inverse:

$-A^{*} = \begin{bmatrix} -1 & 3+ i & -2 - i \\ -3+i &-1 & 5i \\ 2-i &5i & -1\end{bmatrix}$

$A \ne -A^{*}$ , so is not skew-Hermitian.

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Example Question #31 : Symmetric Matrices

$A= \begin{bmatrix} 0 & 3+ i & -2 - i \\ -3+i &0 & 5i \\ 2-i &5i & 0\end{bmatrix}$

True or false: is a skew-Hermitian matrix.

Possible Answers:

False

True

Correct answer:

True

Explanation:

is a skew-Hermitian matrix if it is equal to the additive inverse of its conjugate transpose - that is, if

$A = -A^{*}$

To determine whether this is the case, first, find the transpose of by exchanging rows with columns in :

$A= \begin{bmatrix} 0& 3+ i & -2 - i \\ -3+i &0 & 5i \\ 2-i &5i &0\end{bmatrix}$

$A^{T}= \begin{bmatrix} 0 & -3+i & 2-i \\ 3+ i &0 & 5i \\ -2 - i &5i & 0\end{bmatrix}$

Obtain the conjugate transpose by changing each element to its complex conjugate:

$A^{*}= \begin{bmatrix} 0 &- 3-i & 2+i \\ 3- i &0 & -5i \\ -2 + i &-5i &0\end{bmatrix}$

Now find the additive inverse of this by changing each entry to its additive inverse:

$-A^{*} = \begin{bmatrix} 0& 3+ i & -2 - i \\ -3+i &0 & 5i \\ 2-i &5i & 0\end{bmatrix} = A$

$A = -A^{*}$ , so is skew-Hermitian.

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Example Question #95 : Operations And Properties

$A = \begin{bmatrix} 0 & 4 - i\sqrt{2} \\ x & 0\end{bmatrix}$

Evaluate so that is a skew-Hermitian matrix.

Possible Answers:

$x = -4-i\sqrt{2}$

cannot be made skew-Hermitian regardless of the value of .

$x = -4+i\sqrt{2}$

$x = 4-i\sqrt{2}$

$x = 4+i\sqrt{2}$

Correct answer:

$x = -4-i\sqrt{2}$

Explanation:

is a skew-Hermitian matrix if it is equal to the additive inverse of its conjugate transpose - that is, if

$A = -A^{*}$

Therefore, first, take the transpose of :

$A ^{T}= \begin{bmatrix} 0 &x \\ 4 - i\sqrt{2} & 0\end{bmatrix}$

Obtain the conjugate transpose by changing each element to its complex conjugate:

$A ^{*}= \begin{bmatrix} 0 &\overline{x} \\ 4+ i\sqrt{2} & 0\end{bmatrix}$

Now find the additive inverse of this by changing each entry to its additive inverse:

$-A ^{*}= \begin{bmatrix} 0 &-\overline{x} \\ -4- i\sqrt{2} & 0\end{bmatrix}$

For $A = -A^{*}$ , or,

$\begin{bmatrix} 0 & 4 - i\sqrt{2} \\ x & 0\end{bmatrix}= \begin{bmatrix} 0 &-\overline{x} \\ -4- i\sqrt{2} & 0\end{bmatrix}$ i

It is necessary and sufficient that the two equations

$- \overline{x} = 4-i\sqrt{2}$

and

$x =-4- i\sqrt{2}$

These conditions are equivalent, so

$x =-4- i\sqrt{2}$

makes skew-Hermitian.

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Example Question #33 : Symmetric Matrices

Which of the following matrices is "Skew-symmetric"?

Possible Answers:

$\begin{bmatrix} -1&-5 \\ 5& 1 \end{bmatrix}$

$\begin{bmatrix} -1&5 \\ -5& -1 \end{bmatrix}$

$\begin{bmatrix} 1&5 \\ 5& 1 \end{bmatrix}$

$\begin{bmatrix} 0&5 \\ 5& 0 \end{bmatrix}$

$\begin{bmatrix} 0&-5 \\ 5& 0 \end{bmatrix}$

Correct answer:

$\begin{bmatrix} 0&-5 \\ 5& 0 \end{bmatrix}$

Explanation:

A skew-symmetric matrix is one that becomes negative once the transpose is taken, or A^T=-A .

We have

$\begin{bmatrix} 0&-5 \\ 5& 0 \end{bmatrix}^T =\begin{bmatrix} 0& 5\\ -5& 0 \end{bmatrix} = -\begin{bmatrix} 0&-5 \\ 5& 0 \end{bmatrix}$ .

Hence $\begin{bmatrix} 0&-5 \\ 5& 0 \end{bmatrix}$ is skew-symmetric.

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Linear Algebra : Symmetric Matrices

Study concepts, example questions & explanations for Linear Algebra

All Linear Algebra Resources

Example Questions

Example Question #91 : Operations And Properties

Example Question #31 : Symmetric Matrices

Example Question #32 : Symmetric Matrices

Example Question #31 : Symmetric Matrices

Example Question #95 : Operations And Properties

Example Question #33 : Symmetric Matrices

All Linear Algebra Resources

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