\[
\boxed{
{\frac {{\rm d}^{2}}{{\rm d}{x}^{2}}}y \left( x \right) +f \left( x \right) {\frac {\rm d}{{\rm d}x}}y \left( x \right) + \left( 1/4\, \left( f \left( x \right)  \right) ^{2}+1/2\,{\frac {\rm d}{{\rm d}x}}f \left( x \right) +a \right) y \left( x \right) =0}
\]