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A_lilah
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Homework Statement
Find a matrix B such that B^2 = A
A = 3x3 =
9 -5 3
0 4 3
0 0 1
Homework Equations
B^2 = A
A = XDX^(-1) (similar matrices rule)
also used to find eigenvectors: A - λI
The Attempt at a Solution
Thoughts: If A = XDX^(-1), then B^2 = XDX(-1), and B = X * D^(1/2) * X^-1
So, if I could find D and X for A, I could find B. D = diagonal matrix where the diagonal elements are the eigenvalues of A, and A is lower triangular, so it's eigenvalues are:
λ1 = 9, λ2 = 4, λ3 = 1,
so D = 3x3 =
9 0 0
0 4 0
0 0 1
To find X, I need the eigenvectors:
A-9I =
0 -5 3
0 -5 3
0 0 -8
so (-5)v2 + (3)v3 = 0, or v2 = (3/5)v3
and the eigenvector (representative) = [0 1 3/5]^T
I suspect there is something wrong here because my bottom row in A-9I isn't all 0 and I think it is supposed to be... I don't know what I did wrong though.
For the other eigenvalues, I got eigenvectors that were:
λ2 = 4,
[1 1 0]^T
λ3 = 1
[1 1 -1]
X = 3x3 =
0 1 1
1 1 1
(3/5) 0 -1
X^-1 =
-1 1 0
(8/5) (-3/5) 1
(-3/5) (3/5) -1
To get D^(1/2) I square-rooted the diagonal elements:
3 0 0
0 2 0
0 0 1
Then find X * D^(1/2) * X^-1, which = B, but when I square B, it doesn't = A...
Any input would be great! Thanks
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