\[
\boxed{
{\frac {{\rm d}^{3}}{{\rm d}{x}^{3}}}y \left( x \right) -{x}^{2}{\frac {{\rm d}^{2}}{{\rm d}{x}^{2}}}y \left( x \right) + \left( a+b-1 \right) x{\frac {\rm d}{{\rm d}x}}y \left( x \right) -by \left( x \right) a=0}
\]