Can Eigenvalues and Determinants Determine the Elements of a Square Matrix?

  • Context: Undergrad 
  • Thread starter Thread starter Cygni
  • Start date Start date
  • Tags Tags
    Matrix Square
Click For Summary

Discussion Overview

The discussion revolves around whether knowing the eigenvalues and the determinant of a square matrix can uniquely determine the elements of that matrix. Participants explore the implications of eigenvalues, eigenvectors, and the determinant in relation to matrix identification.

Discussion Character

  • Debate/contested
  • Technical explanation

Main Points Raised

  • One participant questions if knowing the eigenvalues and determinant of a matrix M can lead to determining its elements.
  • Another participant argues that there are infinitely many matrices with the same eigenvalues but different eigenvectors, suggesting that eigenvalues alone do not suffice to identify the matrix.
  • It is proposed that knowing n eigenvalues for an nxn matrix allows for identification of the matrix up to a similarity transformation, but this does not guarantee uniqueness without eigenvectors.
  • A later reply emphasizes that if eigenvectors are known, the matrix can be determined exactly, as changing the order of eigenvalues necessitates changing the order of eigenvectors.
  • One participant acknowledges their prior knowledge of the topic but admits to a lapse in clarity while multitasking.

Areas of Agreement / Disagreement

Participants express differing views on the sufficiency of eigenvalues and determinants for uniquely determining a matrix. Some argue that additional information, such as eigenvectors, is necessary, while others assert that eigenvalues alone lead to a similarity transformation.

Contextual Notes

The discussion does not resolve the nuances of how eigenvalues and eigenvectors interact in determining matrix properties, nor does it clarify the implications of similarity transformations in this context.

Cygni
Messages
38
Reaction score
0
Hello,

Given that we have some matrix M with unknown real elements a, b, c, d and we know its eigenvalues [tex]\lambda_{1}[/tex] and [tex]\lambda_{1}[/tex] (no eigenvectors though) and the value of the determinant is it possible to find the elements and hence the matrix M using this informaiton?

Thanks
 
Physics news on Phys.org
No, there are infinite matrices with the same eigenvalues, but different eigenvectors. For example

[tex]\left(\begin{array}{cc}1&2\\0&3\end{array}\right)\qquad\textrm{and}\qquad\left(\begin{array}{cc}3&4\\0&1\end{array}\right)[/tex]

have the same eigenvalues, but are...different!

By the way, the determinant is just the product of the eigenvalues, so it doesn't give further information.
 
Last edited:
I think that the below is basically correct...

Provided you have n eigenvalues for the nxn matrix, then if you know the eigenvalues then you know the matrix up to a similarity transformation.

If you also know the eigenvectors then that gives you the similarity transformation and thus you know the matrix up to permutation of rows. (provided the eigenvectors are distinct)
 
Simon_Tyler said:
If you also know the eigenvectors then that gives you the similarity transformation and thus you know the matrix up to permutation of rows. (provided the eigenvectors are distinct)

Actually in this case you know the matrix exactly, without any possibility of interchanging rows or columns, because if you change the order of the eigenvalues you have to change the order of the eigenvectors as well, and the result is always the same matrix.
 
@Petr

I knew (and have taught) that!
That should teach me not to post whilst watching tv.
 

Similar threads

  • · Replies 33 ·
2
Replies
33
Views
3K
  • · Replies 5 ·
Replies
5
Views
3K
  • · Replies 1 ·
Replies
1
Views
2K
  • · Replies 14 ·
Replies
14
Views
4K
  • · Replies 4 ·
Replies
4
Views
3K
  • · Replies 2 ·
Replies
2
Views
3K
  • · Replies 24 ·
Replies
24
Views
3K
  • · Replies 20 ·
Replies
20
Views
4K
  • · Replies 6 ·
Replies
6
Views
2K
  • · Replies 1 ·
Replies
1
Views
2K