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Can anyone explain to me why eigenvector here is like this

  1. Mar 29, 2014 #1
    Im supposed to find the eigenvectors and eigenvalues of A

    I found that eigenvalues are 2 12 and -6

    then I found eigen vectors substituting -6 to lambda

    and someone has told me I get 0 0 1 for eigenvector which I cannot understand why??

    can anyone plzzzzzzzzz explain why this is????????

    Attached Files:

    • gag.PNG
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    • gss.PNG
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  2. jcsd
  3. Mar 29, 2014 #2


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    Staff: Mentor

    Did you multiply this vector with your matrix? What do you get?
    Does this fit to the definition of an eigenvector?
  4. Mar 29, 2014 #3
    I get 0 0 0

    it doesn't fit with the definition of eigenvectors

    so does that mean it should be 0 0 0 instead of 0 0 1??

    I mean when you try to calculate for v3 = [ v1 v2 v3]

    since the last row of the matrix is 0 0 0 should v3 come out as 0?? couldn't v3 be any number??
  5. Mar 30, 2014 #4


    Staff: Mentor

    That's not what I get.

    The matrix you showed in the second attachment is not A. It is [A - (-6)I]. Of course if you multiply this matrix times your eigenvector, you'll get the zero vector.
    Assuming that your eigenvalue is λ and that x is an as-yet unknown eigenvector for λ, what you're doing is solving the equation Ax = λx for x. That's equivalent to solving the equation (A - λI)x = 0. In other words, of finding the kernel of the matrix A - λI. This should be something that you have already learned to do.

    The kernel here should not consist of only the zero vector - an eigenvector cannot be the zero vector.
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