\[ y''(x)=-a y(x) \csc ^2(x) \] ✓ Mathematica : cpu = 0.0719046 (sec), leaf count = 70
\[\left \{\left \{y(x)\to c_1 \sqrt [4]{\cos ^2(x)-1} P_{-\frac {1}{2}}^{\frac {1}{2} \sqrt {1-4 a}}(\cos (x))+c_2 \sqrt [4]{\cos ^2(x)-1} Q_{-\frac {1}{2}}^{\frac {1}{2} \sqrt {1-4 a}}(\cos (x))\right \}\right \}\]
✓ Maple : cpu = 0.261 (sec), leaf count = 165
\[ \left \{ y \left ( x \right ) ={{\it \_C1}\sqrt [4]{2\,\cos \left ( 2\,x \right ) +2}{\mbox {$_2$F$_1$}({\frac {1}{4}\sqrt {1-4\,a}}+{\frac {1}{4}},{\frac {1}{4}\sqrt {1-4\,a}}+{\frac {1}{4}};\,{\frac {1}{2}};\,{\frac {\cos \left ( 2\,x \right ) }{2}}+{\frac {1}{2}})}\sqrt {-2\,\cos \left ( 2\,x \right ) +2} \left ( {\frac {\cos \left ( 2\,x \right ) }{2}}-{\frac {1}{2}} \right ) ^{{\frac {1}{4}\sqrt {1-4\,a}}}{\frac {1}{\sqrt {\sin \left ( 2\,x \right ) }}}}+{{\it \_C2} \left ( 2\,\cos \left ( 2\,x \right ) +2 \right ) ^{{\frac {3}{4}}}{\mbox {$_2$F$_1$}({\frac {1}{4}\sqrt {1-4\,a}}+{\frac {3}{4}},{\frac {1}{4}\sqrt {1-4\,a}}+{\frac {3}{4}};\,{\frac {3}{2}};\,{\frac {\cos \left ( 2\,x \right ) }{2}}+{\frac {1}{2}})}\sqrt {-2\,\cos \left ( 2\,x \right ) +2} \left ( {\frac {\cos \left ( 2\,x \right ) }{2}}-{\frac {1}{2}} \right ) ^{{\frac {1}{4}\sqrt {1-4\,a}}}{\frac {1}{\sqrt {\sin \left ( 2\,x \right ) }}}} \right \} \]