Hi. I'm just starting QFT for the first time. I've just finished a course in relativity but i'm confused about the index notation I've found in QFT. Here are 2 examples y(adsbygoogle = window.adsbygoogle || []).push({}); ^{i}= Σ M_{ij}x_{j}and y_{j}= δ_{i}^{j}y^{i}. These examples don't seem right after what I have learnt in relativity unless the index notation changes in QFT ?

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# Index notation in QFT

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