Yes sorry about the 1st. About the 2nd it is used like that in Mandl and my QFT course but I understand that it is confusing. I'd usually use the partial derivatives shorthand, but I wanted it to make sense relative to the screenshot.
So in the 3rd. For example if I lower the second derivative...
I'm currently studying Quantum Field Theory and I have a confusion about some mathematics in page 30 of Mandl's Quantum Field Theory (Wiley 2010).
Here is a screenshot of the relevant part: https://www.dropbox.com/s/fsjnb3kmvmgc9p2/Screenshot%202017-01-24%2018.10.10.png?dl=0
My issue is in...