1. The problem statement, all variables and given/known data Can someone explain what they mean at the bottom of the page "by swapping derivatives with respect to a and x' " and in general how they arrived at equation 3.17? 2. Relevant equations 3. The attempt at a solution
I have to admit that I am a bit puzzled by this. I don't see how a minus sign may be generated even if one relabels the dummy indices, use the chain rules to go from x to c', etc. I must be missing something. I hope someone else can jump in.