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Homework Help Overview

The discussion revolves around properties of matrices in the complex field, specifically focusing on unitary and normal matrices. The original poster presents two statements to prove: that the inverse of a unitary matrix is also unitary, and that the inverse of a normal and invertible matrix is normal.

Discussion Character

  • Exploratory, Mathematical reasoning

Approaches and Questions Raised

  • The original poster attempts to prove the first statement using the property of unitary matrices, questioning if starting with the equation AAT=I is sufficient. They express uncertainty about how to approach the second statement. Other participants engage by suggesting manipulations of the matrix expressions and exploring the implications of the definitions of normal matrices.

Discussion Status

Participants are actively discussing the first part of the problem, with some providing insights into matrix manipulation and properties. There is a clear progression in reasoning, particularly regarding the left-hand side of the equation. The second part remains less developed, with participants questioning whether similar methods apply.

Contextual Notes

The original poster indicates a lack of clarity on how to begin the second proof, suggesting that there may be constraints in their understanding of normal matrices and their properties.

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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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edited this post by mistake
 
Last edited:
so then on the right side would we have (AA*)-1?
 
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)* ?
 
ohh ok so we have (A-1A)* on the left side
 
which is just the identity
 
Correct
 
what about for the 2nd part?
would you start the same?
 
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
 

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