\[
\boxed{
x{\frac {{\rm d}^{3}}{{\rm d}{x}^{3}}}y \left( x \right) +3\,{\frac {{\rm d}^{2}}{{\rm d}{x}^{2}}}y \left( x \right) -a{x}^{2}y \left( x \right) =0}
\]