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Homework Help: Invariant spaces and eigenvector problem

  1. Nov 12, 2012 #1
    1. The problem statement, all variables and given/known data
    Let W be a 1-dimensional subspace of V that is A-invariant. Show that every non zero vector in W is a eigenvector of A. [A element of Mn(F)]

    3. The attempt at a solution

    We know W is A-invariant therefore for all w in W A.w is in W. W is one dimensional which implies to me that A must therefore be a one by one matrix with an entry from F. Is this a correct assumption?

    If so then A.w=λ.w where λ is an element of F which implies that all w in W are eigenvectors of A.

    I'm new to this sort of linear algebra and therefore can't tell if I've made a blatant mistake?
     
  2. jcsd
  3. Nov 12, 2012 #2

    HallsofIvy

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    Science Advisor

    No, in fact it makes no sense at all. Any n by n matrix can have "1-dimensional invarient subspace".

    How did you arrive at that? If w is in the invariant subspace, W, then Aw is also in W and, since W is "1-dimensional", it must be a multiple of A. But it does not immediately follow that that multiple is the same for all vectors in W.

    Suppose that u and v are different vectors in W. Then we can say that [itex]Au= \lambda_1 u[/itex] and that [itex]Av= \lambda v[/itex] but we cannot yet say that [itex]\lambda_1= \lambda_2[/itex]. To do that, we need to use the fact that W is "1-dimensional" again. Because of that, any vector in the subspace is a mutiple of any other (except 0). We can write u= xv so that Au= x(Av). What does that give you?

     
  4. Nov 12, 2012 #3
    Thanks for the help. I'm still a little confused though.

    Why do we need to show that the two λ's are the same. Isn't that [itex] Au= \lambda_1 u[/itex] sufficient because that is the definition of an eigenvector? Unless eigenvalues have to unique for each matrix?
     
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