Linear Algebra : Orthogonal Matrices

Study concepts, example questions & explanations for Linear Algebra

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Example Questions

Example Question #21 : Orthogonal Matrices

Given an orthogonal matrix , does have an inverse?

Possible Answers:

It has no inverse.

Correct answer:

Explanation:

When  is an orthogonal matrix,  certainly has an inverse as well.  has the property:  since it is orthogonal. So we can multiply  by  to get:

so the inverse is 

Example Question #22 : Orthogonal Matrices

True or false::  is an example of an orthogonal matrix.

Possible Answers:

False

True

Correct answer:

False

Explanation:

A matrix  is orthogonal if and only if , the identity matrix, so test this condition.

, its transpose, is found by interchanging rows and columns:

Find  multiplying rows by columns - adding products of entries in corresponding positions:

.

 is not an orthogonal matrix.

Example Question #23 : Orthogonal Matrices

 is an unitary matrix.

True or false:  must be an unitary matrix.

Possible Answers:

False

True

Correct answer:

True

Explanation:

 is an orthogonal matrix, if, by definition, , where  is the conjugate transpose of . Also, for any square matrix , it holds that .

Let  be orthogonal. Since 

it follows that 

Matrix multiplication is associative, so 

By similar reasoning, it can be demonstrated that  is therefore unitary.

Example Question #22 : Orthogonal Matrices

True or false:  is an example of an orthogonal matrix.

Possible Answers:

False

True

Correct answer:

True

Explanation:

A matrix  is orthogonal if and only if 

 is the transpose of . Find this by interchanging rows with columns:,

so

Multiply the matrices by multiplying rows by columns, adding the products of entries in corresponding positions:

 is indeed an orthogonal matrix.

Example Question #21 : Orthogonal Matrices

Is  an orthogonal matrix or a unitary matrix?

Possible Answers:

Unitary

Both 

Orthogonal

Neither

Correct answer:

Unitary

Explanation:

 is an orthogonal matrix if and only if ; it is a unitary matrix if and only if 

, the transpose of , can be found by interchanging rows with columns:

Multiply:

 is not orthogonal.

, the conjugate transpose, can be found by changing each entry in  to its complex conjugate:

Multiply:

 is unitary.

Example Question #24 : Orthogonal Matrices

True or false:  is an example of a unitary matrix.

Possible Answers:

False

True

Correct answer:

True

Explanation:

A matrix is unitary if .

, the conjugate transpose of , can be found by first interchanging rows with columns:

Then changing each entry to its complex conjugate:

Find  by multiplying rows of  by columns, as follows:

 

, so  is a unitary matrix. 

Example Question #22 : Orthogonal Matrices

A real matrix  is both orthogonal and involutory. 

True or false: It follows that  is an identity matrix. 

Possible Answers:

False

True

Correct answer:

False

Explanation:

Real matrix  is orthogonal if and ; it is involutory if . We show the statement is false by giving a nonidentity matrix that has both properties.

Let .

The transpose of , the result of switching rows with columns, is 

It can be seen that , so if and only if . It follows that  is a sufficient condition for  to be both orthogonal and involutory. Square  by multiplying the rows of  by its columns - adding the products of corresponding entries:

\

Thus, there exists at least one non-identity matrix that is both involutory and orthogonal, making the statement false.

Example Question #23 : Orthogonal Matrices

True or false:  is an example of a unitary matrix. 

Possible Answers:

False

True

Correct answer:

True

Explanation:

 is unitary if , where  is its conjugate transpose.  

,

so transpose rows and columns to get 

Now change each entry to its complex conjugate:

Find  by multiplying rows of  by columns of  - adding the products of corresponding entries, as follows:

, so  is unitary. 

Example Question #21 : Orthogonal Matrices

Is the following matrix M orthogonal?

Possible Answers:

It is impossible to determine from the information given.

No, is not orthogonal

Yes, M is orthogonal

Correct answer:

Yes, M is orthogonal

Explanation:

An orthogonal matrix is a real, square matrix that satisfies the following condition:

where I is the identity matrix.

We can calculate the product of M and its transpose:

Therefore, M is indeed an orthogonal matrix.

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