OBS: Ignore factors of ## (2 \pi) ##, interpret any differential ##dx,dp## as ##d^4x,d^4p##, ##\int = \int \int = \int ... \int##. I am using ##x,y,z## instead of ##x_i##.
Honestly, i am a little confused how to show this "triangle-star duality". Look, the propagators in positions space gives...