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If I consider a simple object $X$ in a fusion category and tensor it with its dual $X^*$, and let $Y$ be a simple object in the decomposition $X \otimes X^* = I + Y + \dotsb$. I want to say that $Y$ cannot be pseudo-real, i.e. $Y^{**} = -Y$ (Frobenius-Schur indicator $= -1$), because $(X \otimes X^*)^{**} = (X^{**} \otimes X^*)^* = X^{**} \otimes X^{***}$, and should equal to $X \otimes X^*$. Is this correct?

Perhaps the term "pseudo-real" is not standard in fusion category, but in the case of a $\operatorname{Rep}(G)$ category for some group $G$, my question above becomes the following. Can a pseudo-real representation appear in the decomposition of the tensor product of an irreducible representation $\mathbf r$ with its conjugate representation $\bar{\mathbf r}$?

LSpice
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Ying
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1 Answers1

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If $Y$ appears with multiplicity one, then it cannot be pseudo-real. The proof is that it has a symmetric pairing given by restricting the symmetric pairing on $X \otimes X^*$.

However, if $Y$ appears with even multiplicity, then there are counterexamples, for example see this answer.

Noah Snyder
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