Is the Similarity Matrix S of A always the Identity Matrix?

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Discussion Overview

The discussion revolves around the properties of the similarity matrix S in relation to a matrix A, specifically whether S must always be the identity matrix or if it can take on other forms. The scope includes theoretical considerations of matrix similarity and properties of diagonal and non-diagonal matrices.

Discussion Character

  • Debate/contested
  • Conceptual clarification

Main Points Raised

  • One participant questions if the similarity matrix S must be the identity matrix when A is similar to itself, suggesting that A is nonzero and not the identity matrix.
  • Another participant introduces the case where A is diagonal, implying that S can be non-identity in such scenarios.
  • A later reply agrees that if A is diagonal, S can indeed be non-identity but raises the question of whether S can still be non-identity if A is not diagonal, expressing a belief that it is possible but lacking proof.
  • Another participant provides a specific example of a non-diagonal matrix A and a corresponding similarity matrix S, prompting further exploration of the properties of S.

Areas of Agreement / Disagreement

Participants express differing views on whether S must be the identity matrix, with some suggesting it can be non-identity under certain conditions, particularly when A is diagonal. The discussion remains unresolved regarding the general case of non-diagonal matrices.

Contextual Notes

Participants have not provided proofs for their claims, and there are unresolved questions about the conditions under which S can be non-identity, particularly in relation to the diagonal nature of A.

td21
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for a matrix similar to itself:
[itex]A=SAS^{-1}[/itex]

A is nonzero and not identity.

So is the matrix S must be identity matrix? Can it be non-identity matrix? Thank you.
 
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hi td21! :smile:

(you need to use {} if there's more than one character after ^ or _ :wink:)

what if A is diagonal? :wink:
 
tiny-tim said:
hi td21! :smile:

(you need to use {} if there's more than one character after ^ or _ :wink:)

what if A is diagonal? :wink:


Thank you very much for answering! Yes, if A is diagonal S can be non-identity!
But what if A is not diagonal? Can S be non-identity?(I believe so, but i cannot give a proof. Is it possible to give a proof?)

Also i believe that in any cases(A being diagonal or not), S has to be diagonal. Is this true?
 
Hi td21! :smile:

What if

[tex]A=\left(\begin{array}{cc} a & b\\ 0 & a\\ \end{array}\right)[/tex]

and

[tex]S=\left(\begin{array}{cc} 1 & 2\\ 0 & 1\\ \end{array}\right)[/tex]
 

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