Equation Icon  (0) Oˆ(Ω) = Oˆ (Ω ) + χOF (Ω )ˆx(Ω), Fˆ(Ω) = Fˆ(0)(Ω ) + χF F(Ω )ˆx(Ω), ˆ ˆx(Ω) = χxx (Ω )[F (Ω ) + G (Ω )], (133 )