I hope to replicate my previous thread in QFT which was started three years ago from reading Srednicki's textbook and solution manual and also the problem book that I read (by some serbian fellow). This time I am planning to read several books, so the the title of this thread is general. Anyway, I'll start with my first question from Bjorken's and Drell's classic fields book. My question refers to pages 153-155: I'll quote the passage: My two questions: 1. How to rigorously derive the two equations for Lorentz scalar and 4 vector in the quote above ? 2. How to rigorously derive equations (16.105) and (16.106)? Thanks in advance, I plan on reading also Boyarkin's book and Hatchinson's and other books in QFT and QCD. Cheers!