Gram-Shmidt getting PtAP is diagonal

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Homework Help Overview

The discussion revolves around the Gram-Schmidt orthogonalization process in the context of linear algebra, specifically regarding the diagonalization of a matrix A through its eigenvectors. Participants explore whether orthonormalizing a basis of eigenvectors can lead to a matrix P such that PTAP is diagonal.

Discussion Character

  • Conceptual clarification, Assumption checking

Approaches and Questions Raised

  • Participants consider the implications of orthonormalizing eigenvectors and question whether the resulting vectors remain eigenvectors. There is a discussion about the conditions under which eigenvectors can be orthogonalized and the potential changes to their properties.

Discussion Status

The conversation is ongoing, with some participants expressing uncertainty about the relationship between orthonormalization and eigenvectors. There is a mix of perspectives, with some suggesting that the Gram-Schmidt process may not preserve the eigenvector property, while others seek clarification on the definitions and processes involved.

Contextual Notes

Participants are grappling with the assumptions related to the orthogonality of eigenvectors, particularly when they correspond to different eigenvalues. The discussion highlights the distinction between normalizing vectors and applying the Gram-Schmidt process.

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Homework Statement



in light of the gram-schmidt orthogonalization process, if A: Rn -> Rn and we have a basis of Rn of eigenvectors of A, can't we just orthonormalize them and get a matrix P such that P-1=PT and thus PTAP is diagonal?

Homework Equations





The Attempt at a Solution


I believe the answer to this is yes, but I'm not sure how to prove this. Any ideas would be appreciated, thanks!
 
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nautolian said:

Homework Statement



in light of the gram-schmidt orthogonalization process, if A: Rn -> Rn and we have a basis of Rn of eigenvectors of A, can't we just orthonormalize them and get a matrix P such that P-1=PT and thus PTAP is diagonal?

Homework Equations


The Attempt at a Solution


I believe the answer to this is yes, but I'm not sure how to prove this. Any ideas would be appreciated, thanks!

No, not true. If your eigenvectors have different eigenvalues and aren't already orthogonal, you can apply gram-schmidt but the results won't be eigenvectors.
 
Why aren't those eigenvectors? Because you've changed the vectors too much?
 
nautolian said:
Why aren't those eigenvectors? Because you've changed the vectors too much?

Sure, you changed them too much. Why would you think they would be eigenvectors? A linear combination of two eigenvectors is not necessarily an eigenvector.
 
I thought it might be an eigenvector because i was under the impression for some reason that the orthonormalization would no necessarily mean using the Gram-Shmidt process, but rather just normalizing it (dividing by sqrt(sum of squares of eigenvector values)). Is this not the case?
 
nautolian said:
I thought it might be an eigenvector because i was under the impression for some reason that the orthonormalization would no necessarily mean using the Gram-Shmidt process, but rather just normalizing it (dividing by sqrt(sum of squares of eigenvector values)). Is this not the case?

That's pretty confused. Look, take the matrix [[1,1],[0,2]]. The eigenvalues are 1 and 2. Find the eigenvectors. It's easy. Now you can normalize them, but you can't force them to be orthogonal.
 

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