Orthogonal and symmetric matrices

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Discussion Overview

The discussion revolves around the properties of matrices, specifically focusing on how to construct a matrix that is both symmetric and orthogonal, as well as the distinction between symmetric and unitary matrices. The scope includes theoretical exploration of matrix properties and definitions.

Discussion Character

  • Exploratory, Technical explanation, Debate/contested

Main Points Raised

  • One participant suggests that a symmetric matrix can be formed from any matrix X by computing X + X^{T}.
  • Another participant proposes that if M is a real matrix, the only matrix that is both symmetric and orthogonal is the identity matrix.
  • A different participant adds that a diagonal matrix with 1 or -1 on the diagonal also satisfies the conditions of being symmetric and orthogonal.
  • A later reply clarifies the original question, stating the need for a matrix that is symmetric (not Hermitian) and unitary, defined by M = M^{T} and MM^{\dagger} = I.

Areas of Agreement / Disagreement

Participants express differing views on the types of matrices that meet the criteria of being both symmetric and orthogonal, and the discussion remains unresolved regarding the broader question of symmetric and unitary matrices.

Contextual Notes

The discussion highlights the distinction between symmetric and Hermitian matrices, as well as the definitions of orthogonal and unitary matrices, which may depend on the context of real versus complex matrices.

mnb96
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Hello,
I guess this is a basic question.
Let´s say that If I am given a matrix X it is possible to form a symmetric matrix by computing X+X^{T}.

But how can I form a matrix which is both symmetric and orthogonal? That is:
M=M^{T}=M^{-1}.
 
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You have implicitly stated that M is real. In this case I think only the identity matrix matches your requirements.
 
And also diagonal matrix with 1 or -1 at diagonal. Any more?
 
Thanks for the answers.
I just noticed that unfortunately I stated my problem incorrectly.

Starting from a matrix, I wanted to find another matrix which is symmetric (not Hermitian!) and unitary. That is:

M=M^{T}
MM^\dagger=I

Here M^{T} means "transpose", while M^\dagger means "conjugate transpose".
 

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