1. The problem statement, all variables and given/known data 2. Relevant equations Relabelling of indeces, 4-vector notation 3. The attempt at a solution The forth line where I've circled one of the componants in red, I am unsure why you can simply let ν=μ and μ=v for the second part of the line only then relate it to the first part and add them. If you are chosing "dummy indices" as suggested in the image then wouldn't they have to be independant of the first parts indices? I'm unsure of what is going on here, any ideas would be really helpful, thanks.