I have been looking through some notes on fermion wavefunction operators and noticed some summations involving indexes repeated 3 times.I know this is not allowed when using the Einstein summation convention. So my question is : is the Einstein convention not used in Quantum mechanics ? and do situations ever arise in relativity when an index appears 3 times and what then happens to the convention ?(adsbygoogle = window.adsbygoogle || []).push({});

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# Einstein summation notation

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