Eigenvalues/vectors of Hermitian and corresponding unitary

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Discussion Overview

The discussion centers on the relationship between the eigenvalues and eigenvectors of a Hermitian matrix and those of a corresponding unitary matrix derived from it. The scope includes theoretical exploration of linear algebra concepts, particularly in the context of matrix transformations.

Discussion Character

  • Exploratory, Technical explanation, Conceptual clarification

Main Points Raised

  • One participant states that any Hermitian matrix M can be transformed into a unitary matrix K = U†MU, where U is a unitary matrix.
  • Another participant claims that since the determinant of the unitary matrix U and its adjoint is 1, the eigenvalues of K and M are the same, as shown by det(K - λ) = det(M - λ).
  • A subsequent participant questions whether this implies that K and M have the same eigenvectors.
  • It is noted that the answer depends on the interpretation of "the same" eigenvectors, suggesting that U acts as a change of basis operator, leading to eigenvectors of K being expressed in a different basis than those of M.
  • One participant expresses appreciation for the clarification regarding the change of basis perspective.

Areas of Agreement / Disagreement

Participants generally agree on the relationship of eigenvalues between the Hermitian and unitary matrices but express differing views on the nature of the eigenvectors, indicating that the discussion remains unresolved regarding their equivalence.

Contextual Notes

The discussion does not resolve the implications of eigenvector representation under basis transformation, leaving open questions about the specific forms of the eigenvectors in different bases.

nomadreid
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Given that any Hermitian matrix M can be transformed into a unitary matrix K = UMU, for some unitary U, where U is the adjoint of U, what is the relationship (if any) between the eigenvectors and eigenvalues (if any) of the Hermitian matrix M and the eigenvectors and eigenvalues (if any) of the corresponding unitary matrix K?
 
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The unitary matrix U, and so its adjoint, has determinant 1 so det(K- \lambda)= det(M-\lambda). That shows that they have the same eigenvalues.
 
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Super. S, does that also show that they then have the same eigenvectors?
 
That depends upon what you mean by "the same". We can think of U as a "change of basis" operator. In that sense, eigenvectors of K are the eigenvectors of A written in a different basis. If you are thinking of the vectors as just "list of numbers", then, no, eigenvectors of K will have different numbers because you are writing the vector in a different basis.
 
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thanks, HallsofIvy. Looking at it that way is very enlightening. That was a great help. :) Understanding is seeping in...
 

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