# theory for solving eigenvector and eigenvalue that are unequal

• Sep 19th 2012, 08:15 AM
nivek0078
theory for solving eigenvector and eigenvalue that are unequal
The question asks can a vector X be an eigenvector for two unequal eigenvalues E1 and E2 for the same matrix A? If yes prove it with an example. If No explain why?

I'm really confused on this and would greatly take any and all input. Thank you.
• Sep 19th 2012, 09:52 AM
Deveno
Re: theory for solving eigenvector and eigenvalue that are unequal
if x is an eigenvector for the matrix A, then Ax = λ1x, for some scalar λ1.

if x also satisfies Ax = λ2x, then we have: 0 = Ax - Ax = λ1x - λ2x = (λ12)x.

now eigenvectors cannot be 0 (by definition), so (λ12)x = 0 implies λ12 = 0: that is, λ1 = λ2.

so an eigenvector can only have ONE eigenvalue it belongs to.

it is possible, however, for an eigenvalue to have TWO (or more, linearly independent) eigenvectors.

consider the matrix I =

[1 0]
[0 1].

it should be clear that both (1,0) and (0,1) are eigenvectors with the same eigenvalue, 1, and that these two vectors are linearly independent.
• Sep 19th 2012, 11:18 AM
nivek0078
Re: theory for solving eigenvector and eigenvalue that are unequal
Thank you Deveno! that makes sense now. It gets pretty confusing when you're looking at it for a few hours.

My next question is actually along the same lines. The question states: Can a non-zero vector X be an eigenvector for two unequal eigenvalues E1 and E2 corresponding to two different matrices A and B, respectively? If yes prove it with an example. If No explain why?
• Sep 19th 2012, 12:08 PM
HallsofIvy
Re: theory for solving eigenvector and eigenvalue that are unequal
This is pretty close to trivial. Let A and B be 2 by 2 diagonal matrices with different numbers along the diagonal.
• Sep 23rd 2012, 07:41 AM
nivek0078
Re: theory for solving eigenvector and eigenvalue that are unequal
The question I stated above question where it states: Can a non-zero vector X be an eigenvector for two unequal eigenvalues E1 and E2 corresponding to two different matrices A and B, respectively? If yes prove it with an example. If No explain why?

I've setup a few matrices and I keep getting a result of zero. Can anyone give me a better example of this?
• Sep 24th 2012, 05:42 AM
nivek0078
Re: theory for solving eigenvector and eigenvalue that are unequal
Still not getting the proper answer to the question: Can a non-zero vector X be an eigenvector for two unequal eigenvalues E1 and E2 corresponding to two different matrices A and B, respectively? If yes prove it with an example. If No explain why? Can someone please help! Thank you again in advance for your time.