Showing two matrices are not unitarily similar

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Discussion Overview

The discussion revolves around demonstrating that two specific matrices, A and B, are not unitarily similar. This involves exploring the properties of the matrices and the implications of unitary transformations on their characteristics.

Discussion Character

  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant suggests that if B is expressed as B = UAU^-1 (where U is unitary), then certain properties must hold for A and B.
  • Another participant notes that since B is symmetric (B = B*), this leads to the conclusion that A must also be symmetric (A = A*), which contradicts the known properties of A.
  • A different perspective highlights that the columns of B are orthogonal, and since unitary transformations preserve orthogonality, the non-orthogonality of the columns of A indicates they cannot be unitarily equivalent.
  • One participant raises a point about the definition of "unitary," suggesting that it may depend on whether one considers inner product properties or matrix properties, indicating a deeper connection between these concepts.

Areas of Agreement / Disagreement

Participants express differing views on the implications of unitary similarity and the definitions involved. There is no consensus on a definitive method to show that A and B are not unitarily similar, and the discussion remains unresolved.

Contextual Notes

Participants acknowledge that the definitions of unitary similarity may vary based on the context of inner products versus matrix properties, which could affect the conclusions drawn.

chuy52506
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Let A=
|1 2|
|0 3|
and B=
|1 0|
|0 3|
Show that A and B are not unitarily similar?
 
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anyone have an idea of how to do this?
 
if B = UAU^-1 = UAU* (where U is unitary), then

B* = (UAU*)* = (U*)*A*U* = UA*U*.

but B is symmetric so B = B*. hence we would have

UAU* = UA*U*, and so

U*UAU*U = U*UA*U*U and

A = A*, implying A is symmetric as well, which is false.
 
The same thing said in another way: The columns of B are orthogonal, unitary transformation preserves orthogonality, but the columns of A are not orthogonal. Therefore A and B are not unitarily equivalent.
 
indeed, it all depends on how you define "unitary", by inner product properties, or matrix properties. there is a deep connection between inner products and matrix multiplication.
 

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