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In summary, the conversation discusses how to prove that the inverse of a unitary matrix is also unitary, and how to prove that the inverse of a normal and invertible matrix is also normal. The conversation provides a step-by-step approach for proving both statements using the properties of conjugate transpose and the identity matrix.
  • #1
chuy52506
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0

Homework Statement


A is a matrix in the complex field
Suppose A is unitary show that A-1 is unitary.

Suppose A is normal and invertible, show A-1 is normal.



Homework Equations





The Attempt at a Solution



Can i prove the first one just by:
AAT=I
then AT=A-1

Then
I=A-1(AT)-1
So,
I=A-1(A-1)T


I have no idea in how to start the second one? please help
 
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  • #2
edited this post by mistake
 
Last edited:
  • #3
so then on the right side would we have (AA*)-1?
 
  • #4
So we have A*(A-1)* = A-1(A-1)*

The right side is already in the form that we want. Look at the left side. We want the left side to eventually turn into the identity. If you ignore this problem for a second, given two random matrices P and Q, what's another way of writing Q*P* ? If you're still not sure, take a look at the wiki page:

http://en.wikipedia.org/wiki/Conjugate_transpose

Once you have this figured out, how can we rewrite A*(A-1)* ?
 
  • #5
ohh ok so we have (A-1A)* on the left side
 
  • #6
which is just the identity
 
  • #7
Correct
 
  • #8
what about for the 2nd part?
would you start the same?
 
  • #9
well normal matrix is given by A*A=AA*

consider multiplying on the left by (A*)^-1A^-1 and on the right by A^-1 (A*)^-1
 

1. What is a unitary matrix inverse?

A unitary matrix inverse is the inverse of a unitary matrix. A unitary matrix is a square matrix whose conjugate transpose is equal to its inverse. This means that multiplying a unitary matrix by its inverse will result in the identity matrix.

2. How is a unitary matrix inverse calculated?

The unitary matrix inverse is calculated by using the conjugate transpose of the original matrix. This means that the elements of the original matrix are replaced with their complex conjugates and then the rows and columns are swapped. The resulting matrix is then multiplied by the reciprocal of the determinant of the original matrix.

3. What is the significance of a unitary matrix inverse in linear algebra?

Unitary matrix inverses are important in linear algebra because they can be used to solve systems of linear equations. They also have many useful properties, such as preserving the length of vectors and orthogonality. In quantum mechanics, unitary matrices are used to represent operators that preserve the normalization of wave functions.

4. Are all unitary matrices invertible?

Yes, all unitary matrices are invertible. This is because the determinant of a unitary matrix is always equal to 1 or -1, which means that the matrix is non-singular and has an inverse.

5. Can a unitary matrix inverse be non-square?

No, a unitary matrix inverse must be a square matrix. This is because the inverse of a matrix can only be calculated for square matrices, and a unitary matrix must be square in order to have a conjugate transpose.

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