In Lorentz group in QFT, why the trace of the symmetric second-Rank tensor [tex]S^{\mu\nu}[/tex] is defined as follows?(adsbygoogle = window.adsbygoogle || []).push({});

[tex] S=g_{\mu\nu}S^{\mu\nu}[/tex].

Is it just a definition or the genuine trace of the second-Rank tensor, and why?

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# What is the trace of the second-Rank tensor?

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