Symmetric Matrix and Definiteness

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Homework Help Overview

The discussion revolves around the properties of a symmetric matrix A and the definiteness of its square, A^2. Participants explore the implications of eigenvalues and matrix transformations related to definiteness.

Discussion Character

  • Conceptual clarification, Assumption checking

Approaches and Questions Raised

  • Participants discuss the relationship between the eigenvalues of A and the definiteness of A^2, questioning whether A^2 can be definitively classified as positive definite or semidefinite based on the properties of A.

Discussion Status

The discussion includes various interpretations of the definiteness of A^2, with some participants suggesting that A^2 is positive semidefinite unless A is invertible, while others assert that A^2 is always positive definite except when A is the zero matrix. There is no explicit consensus on the definitive classification of A^2.

Contextual Notes

Participants are considering the implications of the eigenvalue properties of symmetric matrices and the role of invertibility in determining definiteness. There is uncertainty regarding the influence of the orthogonal matrix in the transformation of A.

MikeDietrich
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Homework Statement


If A is a symmetric matrix, what can you say about the definiteness of A^2? Explain.



Homework Equations


I believe I need to use the face that A^2=SD^2S^-1.

I know that if all the eigenvalues of a symmetric matrix are positive then the matrix is positive definiteness and if the eigenvalues are positive and zero then the matrix is semidefinite.

Not sure where to go from here.
 
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So, we need to show that A2 is positive definite? What does this mean?
 
I don't know if A^2 will be positive definite. Oh, wait... if A^2= SD^2S^-1 then the diagonal matrix squared will only give positive or zero eigenvalues so A^2 will be positive semidefinite unless A is invertible then it would be positive definite. Do I have to worry about the orthogonal matrix and its inverse?
 
Yes, A2 will always be positive definite, unless A is zero...
 

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