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I would like to get some information on this topic. It is not discussed in many places, so if any members here know about it, i would be interested in a brief explanation. Or any books or online documents where it is discussed.

D is the "invariant propagation function" or the "propagator". I have read somewhere that

Tr D = 1/(E - E

So the energy levels are poles in the trace of the propagator.

D is the "invariant propagation function" or the "propagator". I have read somewhere that

Tr D = 1/(E - E

_{1}) + 1/(E - E_{2}) + ...So the energy levels are poles in the trace of the propagator.

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