Solving Complex Matrix Problems: Normality, Diagonality, and Unitary Matrices

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Discussion Overview

The discussion revolves around solving complex matrix problems involving Hermitian matrices, specifically focusing on the properties of normality, diagonalization, and unitary matrices. Participants explore the implications of these properties in the context of specific matrix equations and transformations.

Discussion Character

  • Technical explanation
  • Mathematical reasoning
  • Debate/contested

Main Points Raised

  • One participant asserts that N is normal based on the properties of Hermitian matrices A and B.
  • Another participant suggests that A can be recovered from N, and questions whether U diagonalizes N* if it diagonalizes N.
  • It is noted that U*NU can be expressed as U*AU + iU*BU, leading to the conclusion that non-diagonal elements must cancel for both U*AU and iU*BU.
  • A participant challenges the assumption that the sum of two matrices being diagonal implies that both matrices must also be diagonal, raising concerns about the implications of non-diagonal element cancellation.
  • One participant expresses skepticism about Hall's method, arguing that it does not utilize the specific properties of A, B, and N, suggesting that it may not be universally applicable.
  • It is pointed out that if U*NU is diagonal, then (U*NU)* must also be diagonal, and that U*AU can be recovered from the two diagonal matrices derived from part 2.

Areas of Agreement / Disagreement

Participants express differing views on the implications of matrix properties and the validity of certain methods. There is no consensus on the conclusions regarding the diagonalization of U*AU and U*BU, and the discussion remains unresolved regarding the implications of non-diagonal elements.

Contextual Notes

Participants highlight the importance of the specific properties of Hermitian matrices and the conditions under which certain conclusions can be drawn. There are unresolved questions regarding the assumptions made in the application of Hall's method and the implications of matrix element cancellation.

broegger
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Let A and B be Hermitian matrices with AB = BA and let N = A + iB.

1) Show that N is normal.

2) Show that A = 1/2(N+N*) (* = conjugate transpose) and find a formula for B.

3) Let U be a unitary matrix such that U*NU is a diagonal matrix. Show that U*AU and U*BU is diagonal matrices.

I had no problems with 1) and 2) but I simply can't figure out 3)... Please help.
 
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You can recover A form N, and if U diagonalizes N, does it diagonalize N*?
 
Clearly, U*NU= U*AU+ i U*BU. Since U*NU is a diagonal matrix, all non-diagonal elements are 0. That is, All non-diagonal elements of U*AU and iU*BU must cancel. What does that tell you about them individually (and don't forget the "i").
 
Clearly, U*NU= U*AU+ i U*BU. Since U*NU is a diagonal matrix, all non-diagonal elements are 0. That is, All non-diagonal elements of U*AU and iU*BU must cancel. What does that tell you about them individually (and don't forget the "i").

I honorstly don't know... I don't think the fact that two matrices P and Q sum up to a diagonalmatrix D implies that they are diagonalmatrices themselves - it just means that their non-diagonal elements cancel - as you say yourself... Or what?
 
I don't think Hall's method works since it doesn't use at any point the properties of A, B and N, and would thus appear to be 'true' for all matrices, which isn't possible.

However, U*NU diagonal implies (U*NU)*=U*N*U is diagonal, and you may recover U*AU from these two diagonal matrices using part 2
 

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