Null space and eigenspace of diagonal matrix

In summary, the conversation discusses a problem involving a diagonal matrix and finding the eigenspace for a particular eigenvalue. The solution involves multiplying the matrix by certain vectors to determine their eigenvalues. The null space of a diagonal matrix consists of vectors with zero eigenvalues. The eigenspace of an eigenvalue is the null space of A-aI, where A is the given matrix and a is the eigenvalue. The conversation also clarifies the concept of "coming out with" a diagonal matrix and discusses how to find the eigenspace in this scenario.
  • #1
newgrad
2
0

Homework Statement



I am working on a problem where I made a matrix representation of a linear transformation and I am asked what is the eigenspace for a particular eigenvalue.



Homework Equations





The Attempt at a Solution


The problem for me is, I came out with a diagonal matrix. what is the null space of a diagonal matrix? Is it just the 0 vector? If so, can I have an eigenspace?
 
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  • #2
newgrad said:

Homework Statement



I am working on a problem where I made a matrix representation of a linear transformation and I am asked what is the eigenspace for a particular eigenvalue.



Homework Equations





The Attempt at a Solution


The problem for me is, I came out with a diagonal matrix. what is the null space of a diagonal matrix? Is it just the 0 vector? If so, can I have an eigenspace?

Suppose your diagonal matrix is 3x3. Think about what happens when you multiply your matrix by the vectors [1,0,0], [0,1,0] and [0,0,1]. Aren't those all eigenvectors? What are their eigenvalues in terms of the diagonal matrix entries? A null space is just the set of vectors that have zero eigenvalue, right?
 
  • #3
The eigenvalues of a diagonal matrix are on the diagonal.The eigenspace of an eigenvalue a is the nullspace of A-aI,that is,the solutions of (A-aI)x=0
 
  • #4
What do you mean by "came out with" a diagonal matrix? IF the matrix you are dealing with is diagonal, then its eigenvalues are the numbers on the diagonal as Hedipaldi said. An if is the diagonal number in the ith row, its eigenspace is spanned by the ith column. (If the same eigenvalue appears k times on the diagonal, the eigenspace is the space spanned by those k columns.)

But is "came out with" a diagonal matrix means you derived a diagonal matrix somehow from the given matrix we would have to know how it was derived.
 
  • #5
thanks. I think I am seeing the light. So when I subtract off the eigenvalue times I, then i get a 0 in that spot, so the eigenvector can have a 1, or anything, in that corresponding spot. so the eigenspace would be all the vectors with the basis of all O's and a 1 in a column, and the one is where that particular eigenvalue was. did that make sense?
 
  • #6
newgrad said:
thanks. I think I am seeing the light. So when I subtract off the eigenvalue times I, then i get a 0 in that spot, so the eigenvector can have a 1, or anything, in that corresponding spot. so the eigenspace would be all the vectors with the basis of all O's and a 1 in a column, and the one is where that particular eigenvalue was. did that make sense?

Not really. That may not be the clearest explanation. But if it's making sense to you, that's what counts.
 
Last edited:

1. What is a null space of a diagonal matrix?

The null space of a diagonal matrix is the set of all vectors that, when multiplied by the matrix, result in a zero vector. In other words, it is the set of all solutions to the equation Ax = 0, where A is the diagonal matrix.

2. How is the null space of a diagonal matrix related to its eigenvalues?

The null space of a diagonal matrix is closely related to its eigenvalues. The eigenvalues of a diagonal matrix are the entries along the main diagonal, and any vector in the null space of the matrix must contain a 0 in the corresponding position to each eigenvalue. This means that the eigenvalues of a diagonal matrix are also the roots of the characteristic polynomial of the matrix.

3. Can a diagonal matrix have a non-zero null space?

Yes, a diagonal matrix can have a non-zero null space. This occurs when the matrix contains at least one 0 along the main diagonal, which means that there is at least one eigenvalue of 0. This results in a null space that contains all vectors with a 0 in the corresponding position.

4. What is an eigenspace of a diagonal matrix?

An eigenspace of a diagonal matrix is a set of all vectors that, when multiplied by the matrix, result in a scalar multiple of the original vector. In other words, it is the set of all solutions to the equation Ax = λx, where A is the diagonal matrix and λ is an eigenvalue of the matrix.

5. How do you find the null space and eigenspace of a diagonal matrix?

To find the null space of a diagonal matrix, simply set up and solve the equation Ax = 0, where A is the matrix. This will result in a set of vectors that make up the null space. To find the eigenspace, set up and solve the equation Ax = λx, where A is the matrix and λ is an eigenvalue. This will result in a set of vectors that make up the eigenspace.

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