\[ y''(x)=-\frac {a y(x)}{x^4} \] ✓ Mathematica : cpu = 0.0436904 (sec), leaf count = 51
\[\left \{\left \{y(x)\to c_1 x e^{\frac {\sqrt {-a}}{x}}+\frac {c_2 x e^{-\frac {\sqrt {-a}}{x}}}{2 \sqrt {-a}}\right \}\right \}\]
✓ Maple : cpu = 0.027 (sec), leaf count = 31
\[ \left \{ y \left ( x \right ) =x \left ( \cosh \left ( {\frac {1}{x}\sqrt {-a}} \right ) {\it \_C2}+\sinh \left ( {\frac {1}{x}\sqrt {-a}} \right ) {\it \_C1} \right ) \right \} \]