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