Do Diagonal Elements in Congruence Transformations Represent Eigenvalues?

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SUMMARY

Diagonal elements in a congruence transformation can represent the eigenvalues of matrix V under specific conditions related to the transformation matrix A. When A is orthogonal, this relationship is straightforward; however, for non-orthogonal A, certain conditions must be met. Specifically, the determinant and trace of V must equal those of the diagonal matrix D formed by the transformation, leading to the conclusion that det(A) must equal 1 or -1. This establishes a necessary condition for the diagonalization of V through congruence transformations.

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thoughtgaze
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If A diagonalizes V under a congruence transformation (instead of a similarity transformation), are the diagonal elements, the eigenvalues of V?

If A is orthogonal, it is obvious that this is true, but what if A is not orthogonal?
 
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First glance:

There are some conditions on A to allow diagonal of D=A'VA be composed of eigenvalues of V. If this is the case, V and D have the same determinant and trace (first is a product of eig.values and the second is the sum). Same determinant implies
det(A'VA) = det(V),
which means det(A) =1 or det(A)=-1. Same trace implies
tr(V)=tr(A'VA)=tr(AA'V).
I don't immediately see what does this condition mean for A.
 

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