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where <math> \eta </math> are external probing fields coupled to the system and <math>g</math> is the ground state, i.e., an eigenstate of the GTO picked up from the set of eigenstates with the lowest real part of their eigenvalues, a requirement needed to ensure stability of the response. The ground state represents the system which has been allowed to evolve for a long time without perturbations. The generating functional describes how the ground state responses to external perturbations.
When TS is spontaneously broken, the ground state is degenerate
<ref>{{cite book
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