RESTRICTIONS ON GROUND-STATE AVERAGE VALUES IMPOSED BY TIME-REVERSAL SYMMETRY

Time reversal symmetry is applied in the following cases. (a) On the g round state expectation values of the product of two or three Hermitia n operators in order to get some new rules. depending on their T-symme try the expectation value of the commutator or anticommutator (or a fu nction of them) will vanish. (b) On polarization propagators: by enfor cing a simple time reversal relationship between advanced and retarded propagators the strong condition is found that the operators used mus t have the same time reversal symmetry. This condition generalizes ano ther well known condition which relates the 'reality' of the operators with their spin symmetry. Some examples are given for both applicatio ns.