in momentum space, where "G" is the two-particle Green function ⟨ ⟨ Ω Ω | ϕ ϕ 1 ϕ ϕ 2 ϕ ϕ 3 ϕ ϕ 4 | Ω Ω ⟩ ⟩ , "S" are the free propagators and "K" is an interaction kernel, which contains all possible interactions between the two particles. The crucial step is now, to assume that bound states appear as poles in the Green function. One assumes, that two particles come together and form a bound state with mass "M", this bound state propagates freely, and then the bound state splits in its two constituents again. Therefore, one introduces the Bethe–Salpeter wave function Ψ Ψ = ⟨ ⟨ Ω Ω | ϕ ϕ 1 ϕ ϕ 2 | ψ ψ ⟩ ⟩ , which is a transition amplitude of two constituents ϕ ϕ i into a bound state ψ ψ , and then makes an Ansatz for the Green function in the vicinity of the pole as