Invariant symbol implies existence of singlet representation

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Discussion Overview

The discussion revolves around the implications of the Kronecker delta being an invariant symbol in the context of representation theory, specifically regarding the product of a representation R and its complex conjugate representation. Participants explore the relationship between this property and the existence of a singlet representation, particularly in terms of decomposition into irreducible representations.

Discussion Character

  • Technical explanation, Debate/contested

Main Points Raised

  • One participant questions the meaning of the Kronecker delta as an invariant symbol and its implications for the product of representation R and its complex conjugate leading to a singlet representation with zero matrices.
  • Another participant clarifies that the discussion is focused on the product representation containing a singlet representation when decomposed into irreducible representations, emphasizing that the singlet representation is the trivial representation.
  • A repeated point asserts that the Kronecker delta being invariant should imply that the product representation R ⊗ R̅ can be decomposed into a direct sum of the singlet representation and other irreducible representations.
  • One participant states that being invariant is fundamentally linked to the trivial representation.

Areas of Agreement / Disagreement

Participants express differing views on the implications of the Kronecker delta being invariant and its relationship to the existence of the singlet representation. The discussion remains unresolved regarding the specific interpretations and implications of these concepts.

Contextual Notes

There are unresolved assumptions regarding the definitions of representations and the conditions under which the Kronecker delta is considered invariant. The discussion also highlights the complexity of decomposing representations into irreducible components.

AndrewGRQTF
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I don't understand what the last paragraph of the attached page means. Why does the Kronecker delta being an invariant symbol mean that the product of a representation R and its complex conjugate representation has the singlet representation with all matrices being zero?

Doesn't the number zero always form a trivial one-dimensional representation of any group, because when plugged into the equation ##[T ^a , T ^b] = i f^{\text{abc}} T^c## it trivially satisfies it?
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It is not a matter of the possible existence of a one-dimensional representation. It is about the product representation of R and its complex conjugate containing a singlet representation when decomposed into irreducible representations. The singlet representation is the trivial representation.
 
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Orodruin said:
It is not a matter of the possible existence of a one-dimensional representation. It is about the product representation of R and its complex conjugate containing a singlet representation when decomposed into irreducible representations. The singlet representation is the trivial representation.
Why does the Kronecker being invariant mean that the ##R \otimes \overline{R}## can be decomposed into a direct sum of the singlet representation and other irreps?
 
Because being invariant is the fundamental property of being in the trivial representation.
 
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