= Fermionic time ordering
{title2=$T[A(x)B(y)]=\theta(x^0-y^0)A(x)B(y)-\theta(y^0-x^0)B(y)A(x)$}
For two odd <fermionic operators>, <time ordering> contributes a minus sign whenever the chronological rearrangement exchanges them. For several fields the sign is the <sign of a permutation> restricted to the odd fields. This convention makes the free <Dirac propagator> a <Green function> of the <Dirac equation> with its contact term fixed by the <canonical anticommutation relations>.
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