The Matsubara Green's function of this model, defined by G imp ( τ τ ) = − − ⟨ ⟨ T c ( τ τ ) c † † ( 0 ) ⟩ ⟩ , is entirely determined by the parameters U , μ μ and the so-called hybridization function Δ Δ σ σ ( i ω ω n ) = ∑ ∑ p | V p σ σ | 2 i ω ω n − − ϵ ϵ p , which is the imaginary-time Fourier-transform of Δ Δ σ σ ( τ τ ) .