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Linear transformation and find basis

  1. May 8, 2008 #1
    1. The problem statement, all variables and given/known data
    Define T: R2-->R2 by T(x)=Ax
    Find a basis B for R2 with the property that [T]_B is diagonal.

    0 1
    -3 4

    3. The attempt at a solution
    The eigenvalues of a diagonal matrix are its diagonal entries, so here the eigenvalues are 1, and -3. For eigenvalue=1 I get the basis [1,1] and for eigenvalue -3 I do not get a linearly dependent system. However, accidentally solved the system for eigenvalue=3 and got the basis [1,3] which is supposed to be correct. Can anyone see what I'm doing wrong?
  2. jcsd
  3. May 8, 2008 #2


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    The eigenvalues are 3 and 1. Not -3 and 1. If for -3 you don't get a linearly dependent system doesn't that make you wonder what's going wrong? Start by fixing that. I really don't think you needed to post this question.
    Last edited: May 8, 2008
  4. May 8, 2008 #3
    Oh so I would need to solve for the eigenvalues here, not just look at the diagonals? I think I confused this matrix with a triangular matrix.
  5. May 8, 2008 #4


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    Of course you need to solve for the eigenvalues. I don't know what sort of rule you made up to read them off of the opposite diagonal, but it's clearly not right. Like I said, the fact you didn't get a linearly dependent system should immediately say, that's not an eigenvalue!
  6. May 9, 2008 #5


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    Yes, the eigenvalues of a diagonal matrix the entries on the main diagonal. But this is NOT a diagonal matrix- you are asked to change it into one. Furthermore, -3 and 1 are not on the main diagonal!

    What do MEAN by "the basis [1,3]"? That's a single vector. A basis for a two dimensional vector space consists of two vectors.
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