# Guidance for indexology

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1. Aug 9, 2015

### taishizhiqiu

I recently read that indexology is the art of writing a Lagrangian by just knowing how many dimensions it has and how to contract tensors. I am very interested in this technique, but I cannot find any reference. Can anyone give me a guidance or a reference?

2. Aug 10, 2015

### DEvens

Um... You read this where? You might get a scalar from this. But a Lagrangian has to satisfy a few more conditions than just being a scalar.

3. Aug 10, 2015

### taishizhiqiu

4. Aug 10, 2015

### DEvens

Actually, the first one does help. It talks about the other conditions a Lagrangian must satisfy for an electromagnetic field. That is, you need more than just the dimension and how to contract tensors.

5. Aug 10, 2015

### taishizhiqiu

Oh, I think I didn't express myself clearly.

The first book is where I first read about indexology and that's why I asked such a question.

I basically understand the technique. What I want to know is more details. For example, I don't know why only $\delta_{\alpha\beta}$ and $\epsilon_{\mu\nu\lambda}$ is the only two isotropic tensors and I don't even know what are isotropic tensors. That's why I am here asking for reference.

6. Aug 10, 2015