请用英文润色： Here the terms with order higher than $k^2_n$ are neglected. After inserting the expanded equation into Eq.(\ref{fkn}), Eq.(\ref{asr}) and collecting the terms with same $k_n$ order, we have

:

The terms with orders higher than $k^2_n$ are disregarded. Subsequently, the expanded equation is inserted into Eq.(\ref{fkn}) and Eq.(\ref{asr}), and the terms with the same $k_n$ order are collected.