I am reading Wen's book, QFT of many-body systems ( @Xiao-Gang Wen ). I am a little confused about the orthogonal catastrophe introduced in Chap.5. Below Eq.(5.1.6), it is stated that the influence of the core electron on the conduction electron Green's function is not important...''.
If I understand correctly, it is exactly the (almost) vanishing overlap between the initial and final state wavefunctions of the conduction electrons that causes the orthogonal catastrophe. Why does the asumption that the influence of the core electron on the conduction electron Green's function is not important'' works?