Which is the Correct Jordan Form?

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Discussion Overview

The discussion revolves around the properties and definitions of Jordan forms in linear algebra, specifically addressing why certain matrices are considered Jordan forms while others are not. Participants explore the implications of conventions in textbooks regarding the arrangement of Jordan blocks and eigenvectors.

Discussion Character

  • Debate/contested
  • Technical explanation

Main Points Raised

  • One participant seeks clarification on why only one matrix is listed as a Jordan form in their textbook.
  • Another participant suggests that both matrices appear to be Jordan forms and questions the textbook's stance.
  • Some participants propose that the textbook may require a specific arrangement of Jordan blocks and isolated eigenvectors to define a unique Jordan form, although this is noted as not being standard practice.
  • There is a suggestion to refer to the textbook's definition of a Jordan block matrix for further clarification on the arrangement based on multiplicities.
  • A later reply indicates that rearranging Jordan blocks does not affect the similarity of the matrices, implying that the order of blocks should not be a strict requirement.
  • One participant expresses frustration over the textbook's lack of clarity regarding the importance of block arrangement, labeling the requirement as somewhat unreasonable.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the necessity of the arrangement of Jordan blocks. There are competing views on whether the order of blocks affects the classification of Jordan forms.

Contextual Notes

The discussion highlights potential limitations in the textbook's definitions and conventions, particularly regarding the arrangement of Jordan blocks and their implications for identifying Jordan forms.

arthurhenry
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In the book I am looking only one of the following is listed as a Jordan Form. Could somebody tell me why the other one is not (that is, if I were confused again, which property of a Jordan Form I should check and decide on the correct one)
Thank you

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Hi arthurhenry! :smile:

Those both look like Jordan forms to me. Did the book specifically say that the other one wasn't a Jordan form?
 
It is possible that, in order to make the "Jordan Form" unique", your textbook is requiring that "Jordan blocks" come before "isolated" eigenvectors. But that is not standard.
 
HallsofIvy said:
It is possible that, in order to make the "Jordan Form" unique", your textbook is requiring that "Jordan blocks" come before "isolated" eigenvectors. But that is not standard.

That sounds about right.

Look at the definition in your book of a Jordon block matrix, does he say anything about the placement of Jordan blocks according to their multiplicities?
 
Sorry for the late response, and I thank you all. Yes, I did find such a convention in the book.
So this should mean that I can show those two Jordan Forms are "the same", similar ?
I will try to show that now.
Thank you
 
Yes, because if you rearrange the Jordan blocks, then you just have to rearrange the corresponding eigenvectors in the similiarity matrix, so this definition where he requires the Jordan blocks to be placed in a certain order is a bit stupid for this exact reason.

Fine, if it lends itself to a certain formalism but he should have made it clear in the text that the order does not matter, and by setting this question, he clearly has failed to do so.
 

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