Equation Icon  ∑ 𝜀n- α α[ (1)] 2 α[ (2)] 2 α [1 2 (1)] 3 n!□ ξ(n) = − 𝜀L h − 𝜀 L h − 𝜀 L 2£ ξ(1)g + £ ξ(1)h + O (𝜀 ). (21.14 ) n>0