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