Prove 2-Norm: A*A = A^2 Math Help

  • Thread starter Thread starter math8
  • Start date Start date
Click For Summary

Homework Help Overview

The discussion revolves around proving a property of the 2-norm related to matrices, specifically the relationship between the 2-norm of the product of a matrix and its conjugate transpose, and the square of the 2-norm of the original matrix.

Discussion Character

  • Exploratory, Assumption checking, Mathematical reasoning

Approaches and Questions Raised

  • Participants are exploring the proof of the inequality \(\left\|A^{*} A \right\|_{2} \geq \left\| A \right\|^{2}_{2}\) and questioning the validity of the statement for different matrix sizes. There is a focus on understanding the definitions of the matrices involved and their properties.

Discussion Status

The conversation includes attempts to clarify the definitions of the matrices and their operations. Some participants have provided partial proofs for one direction of the inequality but express uncertainty about the other direction. There is an ongoing exploration of specific examples, such as the identity matrix, to test the claims.

Contextual Notes

Participants mention that the matrix \(A\) is defined as belonging to \(\textbf{C}^{m\times n}\), and there is a discussion about the implications of matrix size on the validity of the inequalities being considered.

math8
Messages
143
Reaction score
0
How do you prove that

\left\|A^{*} A \right\|_{2}= \left\| A \right\|^{2}_{2} ?

I can prove that \left\|A^{*} A \right\|_{2} \leq \left\| A \right\|^{2}_{2}

but I am not sure how to do it for the other inequality.
 
Physics news on Phys.org


What is A and what is *? For example, it isn't true for 2x2 matrices.
 


I am sorry, let me specify: A \in \textbf{C}^{m\times n} and A^{*} is the conjugate transpose of A.

Is it true that \left\| A^{*}A \right\|_{2} \geq \left\| A \right\|^{2}_{2} ?

If yes, how do you prove this
 


math8 said:
How do you prove that

\left\|A^{*} A \right\|_{2}= \left\| A \right\|^{2}_{2} ?

I can prove that \left\|A^{*} A \right\|_{2} \leq \left\| A \right\|^{2}_{2}

but I am not sure how to do it for the other inequality.

Try it with

A = [[1,2],[3,4]]

Edit: Better yet, try the identity matrix.
 
Last edited:

Similar threads

  • · Replies 105 ·
4
Replies
105
Views
10K
  • · Replies 7 ·
Replies
7
Views
2K
  • · Replies 9 ·
Replies
9
Views
2K
  • · Replies 11 ·
Replies
11
Views
3K
Replies
4
Views
2K
Replies
5
Views
2K
  • · Replies 6 ·
Replies
6
Views
2K
  • · Replies 5 ·
Replies
5
Views
3K
Replies
4
Views
2K
  • · Replies 22 ·
Replies
22
Views
2K