\[ \boxed { \left ( {\frac {{\rm d}^{2}}{{\rm d}{x}^{2}}}y \left ( x \right ) \right ) y \left ( x \right ) -ax=0} \]
Mathematica: cpu = 25.610752 (sec), leaf count = 18 \[ \text {DSolve}\left [y(x) y''(x)-a x=0,y(x),x\right ] \]
Maple: cpu = 2.090 (sec), leaf count = 103 \[ \left \{ y \left ( x \right ) ={\it ODESolStruc} \left ( {\it \_a}\, \left ( {{\rm e}^{\int \!{\it \_b} \left ( {\it \_a} \right ) \,{\rm d}{ \it \_a}+{\it \_C1}}} \right ) ^{{\frac {3}{2}}},[ \left \{ {\frac {\rm d}{{\rm d}{\it \_a}}}{\it \_b} \left ( {\it \_a} \right ) ={\frac { \left ( 3\,{{\it \_a}}^{2}-4\,a \right ) \left ( {\it \_b} \left ( {\it \_a} \right ) \right ) ^{3}}{4\,{\it \_a}}}+2\, \left ( {\it \_b} \left ( {\it \_a} \right ) \right ) ^{2} \right \} , \left \{ {\it \_a}={ y \left ( x \right ) {x}^{-{\frac {3}{2}}}},{\it \_b} \left ( {\it \_a} \right ) =-2\,{\frac {{x}^{3/2}}{-2\,x{\frac {\rm d}{{\rm d}x}}y \left ( x \right ) +3\,y \left ( x \right ) }} \right \} , \left \{ x={ {\rm e}^{\int \!{\it \_b} \left ( {\it \_a} \right ) \,{\rm d}{\it \_a}+ {\it \_C1}}},y \left ( x \right ) ={\it \_a}\, \left ( {{\rm e}^{\int \!{ \it \_b} \left ( {\it \_a} \right ) \,{\rm d}{\it \_a}+{\it \_C1}}} \right ) ^{{\frac {3}{2}}} \right \} ] \right ) \right \} \]