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Quantum Field Theory of Correlated Bose-Einstein condensates: II. Ward-Takahashi Identities and Correlation Functions

We derive Ward-Takahashi identities for correlated Bose-Einstein condensates based on the expressions of the first-order variations $(δΨ,δG)$ due to perturbations obtained in the preceding paper [T. Kita, J. Phys. Soc. Jpn. $\bf 90$, 024001 (2021)] for the condensate wave function $Ψ$ and Green's function $G$. They enable us to obtain several exact results on the density and current correlation functions $K_{νν'}^{}$, and also express $K_{νν'}^{}$ in terms of low-energy Green's functions and vertices. The latter expressions open up the possibility of constructing theory of superfluid Bose liquids in the same way as that for fermions at low temperatures. The vertices are found to have different limits depending on which of frequency $ω$ and wavenumber $q$ is set equal to zero first.