\[
\boxed{
{\frac {{\rm d}^{2}}{{\rm d}{x}^{2}}}y \left( x \right) - \left(  \left( f \left( x \right)  \right) ^{2}+{\frac {\rm d}{{\rm d}x}}f \left( x \right)  \right) y \left( x \right) =0}
\]