QFT is based on an integral instead of a differential approach to the formulation of problems. This means you need a Green ( propagator) function inside the integral. Moreover instead of working in space -time coordinates, Feynman showed the convenience of working with momentum-energy variables...
What I find most troubling in the teaching of calculus (in One dim) is to call attention on the fact the ubiquitous delta x:
a) does not go to zero in the denominator, one divides by deltax before letting deltax tend to zero
b) if a problems calls for the solution of a differential or an...