Equation Icon  ′ 2 ′ ⟨Φk (η)Φk′(η)⟩s = (2π) δ(⃗k + ⃗k ) (174 ) ∫ η ∫ η′ × dη1 dη2Gk (η,η1)Gk ′(η′,η2)⟨(ξ0i)k(η1)(ξ0i)k′(η2)⟩s. (175 )