= Propagator kernels in a positive-sign Gaussian generating functional
{title2=$D=-i\Delta,\quad F=-iS$}
With source term $J\phi+\bar\eta\psi+\bar\psi\eta$ and left <functional derivatives>, a free functional written as $Z_0\propto\exp(iJ\Delta J/2+i\bar\eta S\eta)$ has physical time-ordered contractions $D=-i\Delta$ and $F=-iS$. The kernels in this exponent must not be identified with contractions already carrying factors of $i$. Four contractions in a second-order Yukawa scalar two-point graph introduce $(-i)^4=1$, leaving its kernel formula's overall coefficient unchanged.
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