Abstract
The formalism we have developed so far is limited to zero temperature (i.e., to the ground state) properties of many-body systems. As you remember, this is because the ground state is always nondegenerate, so that we could pull off the trick with adiabatic hypothesis: if you slowly turn interactions on, and then off, the worst that can happen is some phase factor, which anyway cancels. This, in turn, allowed us to build up the diagrammatic technique.
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Such, such were the joys When we all, girls and boys, In our youth time were seen On the Ecchoing Green. William Blake “Songs of Innocence”
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© 1998 Springer Science+Business Media New York
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Zagoskin, A.M. (1998). More Green’s Functions, Equilibrium and Otherwise, and Their Applications. In: Quantum Theory of Many-Body Systems. Graduate Texts in Contemporary Physics. Springer, New York, NY. https://doi.org/10.1007/978-1-4612-0595-1_3
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DOI: https://doi.org/10.1007/978-1-4612-0595-1_3
Publisher Name: Springer, New York, NY
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