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Mmmm
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Homework Statement
I have posted this problem on another website (mathhelpforum) but have received no replies. I don't know whether this is because no one knows what I am talking about or if it's just that no one can find a fault with my reasoning. Please please please could you post a reply even if it's just to say "Looks ok to me, but what would I know?" as there is the (remote) possibility that the book is wrong here and it'd really do me good to know this - the more replies saying it looks ok the happier I'll be. It's playing havok with my confidence.My book (Tensor Geometry - Poston & Dodson) says the following:
If [itex]\beta = (b_1,..., b_n)[/itex] is a basis for X, and [itex]A : X \rightarrow Y[/itex] is an isomorphism, then [itex] A\beta = (Ab_1,..., Ab_n)[/itex] is a basis for Y.
If [itex]\beta[/itex] is a basis for X and [itex]A : X \rightarrow X[/itex] is an isomorphism, the change of basis matrix [itex]_\beta^{A\beta}[/itex] is exactly the matrix [itex]([A]_\beta^\beta)^{-1}[/itex].
I just can't seem to agree with this result!
Homework Equations
The Attempt at a Solution
After hours of tearing my hair out I have come up with the following argument...
For some basis [itex]\beta[/itex], some vector [itex]\mathbf{x}[/itex] and its representation [itex]x^\beta[/itex] in the [itex]\beta[/itex] coords.
[tex]\beta x^\beta=\mathbf{x}[/tex]
[tex]\Rightarrow x^\beta=\beta^{-1}\mathbf{x}[/tex]
[tex]\Rightarrow _\beta^{\beta'} x^\beta=_\beta^{\beta'} \beta^{-1}\mathbf{x}[/tex]
for some other basis [itex]\beta'[/itex] where [itex]_\beta^{\beta'} [/itex] is the change of basis matrix from [itex]\beta[/itex] to [itex]\beta'[/itex] coordinates. so
[tex] _\beta^{\beta'} x^\beta=_\beta^{\beta'} \beta^{-1}\mathbf{x}= x^{\beta'}[/tex] --(*)
We also know the coordinates of [itex]\mathbf{x}[/itex] in the [itex]\beta'[/itex] coords using the [itex]\beta'[/itex] basis:
[tex]\beta' x^{\beta'}=\mathbf{x}[/tex]
[tex]\Rightarrow x^{\beta'}=\beta'^{-1}\mathbf{x}[/tex] --(**)
(*) and (**) combine to give
[tex]_\beta^{\beta'} \beta^{-1}\mathbf{x}=\beta'^{-1}\mathbf{x}[/tex]
[tex]\Rightarrow _\beta^{\beta'} \beta^{-1}=\beta'^{-1} [/tex]
[tex]\Rightarrow _\beta^{\beta'}=\beta'^{-1}\beta[/tex]
This seems like a nice neat result to me, but if [itex]\beta'=A\beta[/itex] as it is in the book, we have
[tex] _\beta^{\beta'}=\beta'^{-1}\beta[/tex]
[tex]\Rightarrow _\beta^{A \beta}=(A\beta)^{-1}\beta[/tex]
[tex]\Rightarrow _\beta^{A \beta}=\beta^{-1}A^{-1}\beta[/tex]
[tex]\not=A^{-1}[/tex]
However, if [itex]\beta'=\beta A[/itex]
[tex] _\beta^{\beta A}=\beta'^{-1}\beta[/tex]
[tex]\Rightarrow _\beta^{\beta A}=(\beta A)^{-1}\beta[/tex]
[tex]\Rightarrow _\beta^{\beta A}=A^{-1}\beta^{-1}\beta[/tex]
[tex]=A^{-1}[/tex]
which is the required result...
I have tried some basic examples with actual numbers and the results support what I have here... Unless I have some fundamental misunderstanding of all this and what it is supposed to mean, which is quite possible...
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