causal propagator | | | | Peierls-Poisson bracket |
advanced propagator | | | | future part of Peierls-Poisson bracket |
retarded propagator | | | | past part of Peierls-Poisson bracket |
Wightman propagator | | | | positive frequency of Peierls-Poisson bracket, Wick algebra-product, 2-point function of vacuum state or generally of Hadamard state |
Feynman propagator | | | | time-ordered product |