Just for the fun of it, prove ...

 If you are ever minded to learn quantum field theory, and chose to do it from the book Quantum Field Theory by Lowell S. Brown, then you are in for a treat, because some of the problems are fun.  For instance:

If you wish to partake in the fun, the Google Books link above has the rest of the question to give context to the symbols in the equation.  

If you want to put a proof in the comments, feel free!