Equation Icon  ρ = ρ0 + 𝜖ρ1 + O (𝜖2), (14 ) 2 p = p0 + 𝜖p1 + O (𝜖 ), (15 ) ϕ = ϕ0 + 𝜖ϕ1 + O (𝜖2). (16 )