How do you orthogonally diagonalize the following matrix:

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- Apr 20th 2009, 08:07 AMantmanOrthogonally diagonalize the matrix
How do you orthogonally diagonalize the following matrix:

- Apr 20th 2009, 08:53 AMTwighi
hi

You need to find the eigenvalues of the matrix, and find the corresponding eigenvectors. Do you know how to do this?

, since A is orthogonally diagonizable, P will be an orthogonal matrix, hence .

An easy way to find the eigenvalues of this matrix would be a cofactor expansion for example, or you could use the fact that it is block triangular.

Itīs quite a long process to describe if you have no idea whatsoever on how to find eigenvalues or eigenvectors, and I would in that case suggest you start there. - Apr 21st 2009, 01:16 PMantman
I think the eigenvalues are 5, 1 and 1 or just 5 and 1? and the eigenvectors are <0 -1 1>, <0 1 1> and <1 0 0>?

- Apr 21st 2009, 01:55 PMTwighi
hi

Yes, the eigenvalus are so the multiplicity of eigenvalue 1 is two. This means that the dimension of the nullspace of the matrix has dimension 2.

The eigenvectors you wrote are correct.

There is a theorem that says that eigenvectors from different eigenspaces to a symmetric matrix are orthogonal, so you only need to verify that the two vectors corresponding to eigenvalue 1 are orthogonal to each other.

Which they are here.

Now you normalize your eigenvectors and put them in a matrix P, and the eigenvalues you put in a diagonal matrix D, with the eigenvalue corresponding to the first column vector in the first column of D etc. Order matters here!

And you are done! - Apr 21st 2009, 02:08 PMantman
My P matrix is a little ugly but I'm hoping it is correct.

- Apr 21st 2009, 02:28 PMantman
and my final would be the eigenvectors on the diagonal like ?

- Apr 22nd 2009, 03:39 AMTwighi
hi

Yes, your matrix P is correct, and so is your matrix D.

I assumed you meant eigenVALUES on the diagonal, not eigenvectors (Happy)

good job