2. Show that the expression
\[
x^{(2)}(t)=x_{0}+\frac{1}{2}\left[x_{\omega} e^{-i \omega t}+x_{\omega}^{*} e^{i \omega t}\right]+\frac{1}{2}\left[x_{2 \omega} e^{-2 i \omega t}+x_{2 \omega}^{*} e^{2 i \omega t}\right]
\]
is a solution of
\[
\frac{d^{2} x}{d t^{2}}+\omega_{0}^{2} x+a x^{2}=\frac{-e}{2 m}\left[E_{\omega} e^{-i \omega t}+E_{\omega}^{*} e^{i \omega t}\right]
\]
that is valid up to second order in the electric-field strength.