 We then use
where formula_105 is the annihilation operator for the single-particle state formula_106 and formula_107 is that state's wavefunction in the position basis.This gives
with a similar expression for formula_109.These depend only on the difference of their time arguments, so that
and

We can again define retarded and advanced functions in the obvious way; these are related to the time-ordered function in the same way as above.