Non-diagonalisable matrix of a linear trans.

In summary: And I've decided to come back too.In summary, the matrix is not diagonalizable if it has a basis of consisting of n eigenvectors.
  • #1
pibeta
3
0

Homework Statement



If the matrix of a linear transformation relative to a basis is non-diagonalisable, then for any other choice of basis, it too will be non-diagonalisable. Prove this is the case.

Homework Equations



Similar matrices.

The Attempt at a Solution



Let APB=[T]BPB, where A is a nxn matrix.

But I'm stuck. Not sure what to do next.
 
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  • #2
What is your definition of diagonalizable and non-diagonalizable? It makes a pretty big difference since there are a couple ways to approach this depending on what we can assume and what we want to prove
 
  • #3
If it's diagonalizable then it has a basis of consisting of n eigenvectors. The choice of the basis to represent the matrix doesn't change that.
 
  • #4
There might be an easier way to show this, but in physics diagonalizing matricies always leads to eigenvalue problems... I believe your statement implies the matrix doesn't have a closed form for its eigenvalue equation. So
[tex]|A-\omega I|=0[/tex] wouldn't have n roots and hence can't be diagonalized.

But as office-shredder says, there is more than one way to diagonalize a matrix...
 
  • #5
Office_Shredder said:
What is your definition of diagonalizable and non-diagonalizable? It makes a pretty big difference since there are a couple ways to approach this depending on what we can assume and what we want to prove

If you mean real diagonalizable vs complex diagonalizable, I'm not sure that's what this is about.
 
  • #6
Dick said:
If you mean real diagonalizable vs complex diagonalizable, I'm not sure that's what this is about.

No, I mean, for example, the definition of non-diagonalizable that I would use, and a lot of other people, is something like "a matrix which is not diagonal under any change of basis". Making the whole question kind of moot. Someone thought this question was worth asking, so obviously there's a different definition floating around.

I would assume that a diagonalisable matrix would have a diagonal and the off-diagonal entries are all zero, and then the non-diagonalisable matrix would have off-diagonal entries not all zero - that's how I would define it (and I'm hoping that's the way my teacher wanted us to define it!)

That would be a diagonal and a... non-diagonal matrix. What is a diagonalizable matrix, and what is a non-diagonalizable matrix? You can't expect to solve the problem without knowing what you're talking about
 
  • #7
Office_Shredder said:
No, I mean, for example, the definition of non-diagonalizable that I would use, and a lot of other people, is something like "a matrix which is not diagonal under any change of basis". Making the whole question kind of moot. Someone thought this question was worth asking, so obviously there's a different definition floating around.

Now you are just confusing me. What does being diagonalizable have to do with the choice of a basis? The problem just says show it's independent of basis. If you state the condition in terms of eigenvectors, it's clearly basis independent.
 
  • #8
Looks like I've caused too much trouble here. Thanks for trying.

Pibeta.
 
  • #9
pibeta said:
Looks like I've caused too much trouble here. Thanks for trying.

Pibeta.

Don't leave! Just show that the quality of a matrix being diagonalizable is independent of the basis by stating it in a way that is independent of the basis. I'm not sure what this flurry of confusion was about, but it's nothing about you causing trouble.
 
  • #10
keniwas said:
There might be an easier way to show this, but in physics diagonalizing matricies always leads to eigenvalue problems... I believe your statement implies the matrix doesn't have a closed form for its eigenvalue equation.
"Doesn't have a closed form for its eigenvalue equation"? What does that mean?

So
[tex]|A-\omega I|=0[/tex] wouldn't have n roots and hence can't be diagonalized.
That doesn't follow. If an operator (on an n dimensional vector space) has n distinct roots then it is diagonalizable, but if not it still may be diagonalizable. As Dick said, it depends on the number of independent eigenvectors, not eigenvalues.

But as office-shredder says, there is more than one way to diagonalize a matrix...
No, office-shredder did not say that. He said there is more than one way to define "diagonalizable".
 
  • #11
Thank you for all of this help (I know what diagonalisable means, but didn't realize they was more than one way to define it). And I've decided to come back too.

Anyways, a hint I was given was to use [T]C=PC<-B[T]B(PC<-B)-1 and prove the contradiction, which is the matrix is diagonalisable.

To show it is diagonalisable, I got (using the hint) [T]C=PBAPB-1 (where A=PB[T]B(PB)-1 )but I'm not sure if this is right and where to go from here...
 
  • #12
pibeta said:
Thank you for all of this help (I know what diagonalisable means, but didn't realize they was more than one way to define it). And I've decided to come back too.

Anyways, a hint I was given was to use [T]C=PC<-B[T]B(PC<-B)-1 and prove the contradiction, which is the matrix is diagonalisable.

To show it is diagonalisable, I got (using the hint) [T]C=PBAPB-1 (where A=PB[T]B(PB)-1 )


but I'm not sure if this is right and where to go from here...
I'm glad to know that you know what "diagonalizable" means- that helps a lot! But to help you we still need to know what your definition of "diagonalizable" is! Please tell us.
 

1. What is a non-diagonalisable matrix?

A non-diagonalisable matrix is a square matrix that cannot be transformed into a diagonal matrix through a similarity transformation. In other words, it cannot be written as a product of a diagonal matrix and an invertible matrix.

2. How do I know if a matrix is non-diagonalisable?

A matrix is non-diagonalisable if it does not have a full set of linearly independent eigenvectors. This can be determined by finding the eigenvalues of the matrix and checking if their corresponding eigenvectors span the entire vector space.

3. Can a non-diagonalisable matrix still have eigenvalues?

Yes, a non-diagonalisable matrix can still have eigenvalues. However, it will not have a full set of linearly independent eigenvectors, which is a requirement for a matrix to be diagonalisable.

4. Why is it important to study non-diagonalisable matrices?

Non-diagonalisable matrices have many important applications in various fields, such as quantum mechanics and control theory. They can also provide insights into the properties of linear transformations and their relationships with vector spaces.

5. How can I determine the Jordan canonical form of a non-diagonalisable matrix?

The Jordan canonical form of a non-diagonalisable matrix can be found by first finding its Jordan normal form, which is a block diagonal matrix with Jordan blocks along the diagonal. The Jordan blocks are then filled in with the appropriate eigenvalues and the resulting matrix is the Jordan canonical form.

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