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Example Questions
Example Question #21 : Orthogonal Matrices
Given an orthogonal matrix , does have an inverse?
It has no inverse.
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.
False
True
False
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.
False
True
True
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.
False
True
True
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?
Unitary
Both
Orthogonal
Neither
Unitary
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.
False
True
True
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.
False
True
False
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.
False
True
True
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?
It is impossible to determine from the information given.
No, M is not orthogonal
Yes, M is orthogonal
Yes, M is orthogonal
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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