\[ \boxed { {\frac {{\rm d}^{2}}{{\rm d}{x}^{2}}}y \left ( x \right ) +{a}^{2}y \left ( x \right ) -\cot \left ( ax \right ) =0} \]
Mathematica: cpu = 0 (sec), leaf count = 0 \[ \text {Hanged} \]
Maple: cpu = 0.032 (sec), leaf count = 39 \[ \left \{ y \left ( x \right ) =\sin \left ( ax \right ) {\it \_C2}+\cos \left ( ax \right ) {\it \_C1}+{\frac {\sin \left ( ax \right ) }{{a}^{2} }\ln \left ( {\frac {\sin \left ( ax \right ) }{\cos \left ( ax \right ) + 1}} \right ) } \right \} \]