Can transformation coefficients be interchanged in symmetric tensors?

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Discussion Overview

The discussion revolves around the interchangeability of transformation coefficients in the context of symmetric tensors, specifically whether the symmetry of tensor components in one frame implies symmetry in another frame after transformation. The scope includes theoretical aspects related to tensor properties and transformation laws.

Discussion Character

  • Homework-related
  • Technical explanation
  • Conceptual clarification

Main Points Raised

  • The original poster (OP) questions the validity of changing the order of transformation coefficients when proving that symmetric tensors remain symmetric under transformation.
  • One participant suggests that since the transformation coefficients are just numbers, the question simplifies to whether multiplication of real numbers is commutative.
  • Another participant asserts that due to the symmetry of the tensor, interchanging indices i and j in the expression is permissible.
  • A later reply reiterates the idea that a tensor with symmetric components in one coordinate system should maintain symmetry in all coordinate systems, emphasizing the exchange of indices p and q in the tensor components.

Areas of Agreement / Disagreement

Participants express some agreement on the implications of symmetry in tensors, but there is uncertainty regarding the specific validity of changing the order of transformation coefficients. The discussion does not reach a consensus on this point.

Contextual Notes

The discussion does not clarify the assumptions regarding the properties of the transformation coefficients or the specific conditions under which the interchangeability holds. There is also no resolution on the mathematical steps involved in the proof presented.

spacetimedude
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Homework Statement


The lecture notes states that if ##T_{ij}=T_{ji}## (symmetric tensor) in frame S, then ##T'_{ij}=T'_{ji}## in frame S'. The proof is shown as $$T'_{ij}=l_{ip}l_{jq}T_{pq}=l_{iq}l_{jp}T_{qp}=l_{jp}l_{iq}T_{pq}=T'_{ji}$$ where relabeling of p<->q was used in the second equality. Where I am confused is that after the 3rd equality, the order of ##l_{iq}## and ##l_{jp}## changes. Is this allowed in all tensors?
Thanks in advance
 
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The ##l_{jp}## are just numbers so your question boils down to "is ##xy = yx##?" where ##x## and ##y## are real numbers.
 
Since the tensor is symmetric then you can interchange the i and j in the expression.

Right?
 
jedishrfu said:
Since the tensor is symmetric then you can interchange the i and j in the expression.

Right?
The idea was to show that a tensor with symmetric components in one coordinate system has symmetric components in all coordinate systems. Above this meand that the exchange of p and q in ##T_{pq}## is assumed to be fine and we want to show that this implies that i and j can be exchanged. The OP's question was regarding the validity of changing the order of the transformation coefficients.
 
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