Matrix Similarity and Eigenvalues

  • Thread starter Thread starter MikeDietrich
  • Start date Start date
  • Tags Tags
    Eigenvalues Matrix
Click For Summary

Homework Help Overview

The discussion revolves around the concept of matrix similarity and eigenvalues, specifically focusing on whether two 3 x 3 matrices with the same eigenvalues must be similar. The original poster presents a statement for evaluation and seeks clarification on the implications of eigenvalues in determining similarity.

Discussion Character

  • Conceptual clarification, Assumption checking

Approaches and Questions Raised

  • The original poster attempts to reason through the relationship between eigenvalues and matrix similarity, noting that while similar matrices share eigenvalues, the reverse may not hold. They provide an example of 2 x 2 matrices to illustrate their point but seek a counterexample for 3 x 3 matrices. Other participants question the diagonalizability of specific matrices and its relevance to the problem at hand.

Discussion Status

The discussion is active, with participants exploring the implications of eigenvalues on diagonalizability and similarity. Some guidance has been offered regarding the diagonalization process and its challenges, indicating a productive exchange of ideas without reaching a definitive conclusion.

Contextual Notes

There is an underlying assumption that the matrices in question are 3 x 3 and that the eigenvalues provided are unique. The original poster expresses difficulty in finding a counterexample, which may suggest constraints in their exploration.

MikeDietrich
Messages
31
Reaction score
0

Homework Statement


If two 3 x 3 matrices A and B have the eigenvalues 1, 2, and 3, then A must be similar to B. True or False and why.

Homework Equations


A is similar to B iff B = S^-1AS

The Attempt at a Solution


I know that if A and B are similar then they have the same eigenvalues but the same does not always hold true the other way. For example, [1 0 ## 0 1] and [1 1 ## 0 1] both have eigenvalues of 1 and 1 but the first is diagonalizable and the second is not so they are not similar. However, I cannot find a counterexample for a 3 x 3 matrix. Any thoughts? Thank you!
 
Physics news on Phys.org
Think about why you can't diagonalize the matrix

\begin{pmatrix}1 & 1 \\ 0 & 1\end{pmatrix}

What problem do you run into when you try to diagonalize it? Will that same problem crop up with A and B?
 
Since the eigenvalues are given and they are 3 unique eigenvalues then there will be an eigenbasis and the matrices will both have to be diagonalizable therefore, the statement is true (am I on the right track?).
 
Yup, you got it.
 

Similar threads

Replies
5
Views
2K
Replies
2
Views
2K
  • · Replies 8 ·
Replies
8
Views
2K
Replies
9
Views
2K
Replies
8
Views
3K
  • · Replies 7 ·
Replies
7
Views
2K
  • · Replies 24 ·
Replies
24
Views
4K
  • · Replies 10 ·
Replies
10
Views
2K
  • · Replies 2 ·
Replies
2
Views
2K
  • · Replies 7 ·
Replies
7
Views
3K