quantum field theory - Wick's theorem and Feynman propagator

(this is the image from book 'No nonsense QFT' by Jakob Schwichtenberg, page no, 426) The quantity $[\phi_-(x),\phi_+(y)]$ is like an operator inside the bra-kets $\langle 0|$ and $|0\rangle$. I'm

homework and exercises - Proof of Wicks Theorem, 3 Fields

a) Feynman rules of the QED field theory. The wavy line denotes

Peeter Joot's Blog » PHY2403H Quantum Field Theory. Lecture 15b

An Introduction To Quantum Field Theory: Peskin, Michael E

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Quantum Field Theory Wick's Theorem

Feynman propagator in nLab

Feynman diagrams potentially contributing to the propagator in

Wick's theorem - Wikiwand

How to calculate the Feynman propagator in quantum field theory