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