Is There a General Way to Format a Counterexample for Diagonal Matrices?

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Discussion Overview

The discussion revolves around the question of whether a non-diagonal matrix can commute with a diagonal matrix under certain conditions, specifically when the product of the two matrices is equal in either order. Participants explore the implications of providing counterexamples to this question and the sufficiency of such examples in disproving the original statement.

Discussion Character

  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • The original poster (OP) questions if a matrix C that commutes with a diagonal matrix A must also be diagonal, citing a specific example where C is a matrix with ones in every entry.
  • Some participants assert that providing one counter-example is sufficient to disprove the statement in question, emphasizing that no additional examples are necessary.
  • There is a discussion about the clarity of communication in responses, with one participant requesting clearer attribution in replies to avoid confusion about whom is being addressed.

Areas of Agreement / Disagreement

Participants generally agree that a single counter-example is sufficient to disprove the original statement. However, there is some disagreement regarding the clarity of communication in the discussion.

Contextual Notes

The discussion does not resolve whether there are additional conditions under which the commutation holds or if there are other forms of counterexamples that could be considered.

jsgoodfella
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If we take an nxn diagonal matrix, and multiply it by an nxn matrix C such that AC=CA, will C be diagonal? I know, for instance, if C is a matrix with ones in every entry, AC=CA holds. But is there a more general way to format such a counterexample, or have I already provided a sufficient "proof"?



Thanks in advance. This isn't a homework question.
 
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jsgoodfella said:
If we take an nxn diagonal matrix, and multiply it by an nxn matrix C such that AC=CA, will C be diagonal? I know, for instance, if C is a matrix with ones in every entry, AC=CA holds. But is there a more general way to format such a counterexample, or have I already provided a sufficient "proof"?



Thanks in advance. This isn't a homework question.


[tex]\left(\begin{array}{cc}2&0\\0&2\end{array}\right) \left(\begin{array}{cc}1&1\\0&1\end{array}\right)=\left(\begin{array}{cc}2&2\\0&2\end{array}\right)=\left(\begin{array}{cc}1&1\\0&1\end{array}\right) \left(\begin{array}{cc}2&0\\0&2\end{array}\right)[/tex]
DonAntonio
 
When you disprove a statement by providing a counter-example, one counter-example is sufficient. There is no need to provide more.
 
phyzguy said:
When you disprove a statement by providing a counter-example, one counter-example is sufficient. There is no need to provide more.



Yes, of course. Whom are you addressing and why?

DonAntonio
 
DonAntonio said:
Yes, of course. Whom are you addressing and why?

DonAntonio

The OP asked whether he had already provided a sufficient proof. The answer is yes - since had already provided one counter-example, this is sufficient to disprove the original statement. That's all I'm saying.
 
phyzguy said:
The OP asked whether he had already provided a sufficient proof. The answer is yes - since had already provided one counter-example, this is sufficient to disprove the original statement. That's all I'm saying.



Good. This time you provide a quote of whom you're addressing and thus we know. The last time we, or at least I, didn't know.

DonAntonio
 

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