\[ \boxed { \left ( {\frac {{\rm d}^{2}}{{\rm d}{x}^{2}}}y \left ( x \right ) \right ) y \left ( x \right ) -a=0} \]
Mathematica: cpu = 0.196025 (sec), leaf count = 115 \[ \left \{\left \{y(x)\to \exp \left (\frac {-c_1-2 a \text {erf}^{-1}\left (-i \sqrt {\frac {2}{\pi }} \sqrt {a e^{\frac {c_1}{a}} \left (c_2+x\right ){}^2}\right ){}^2}{2 a}\right )\right \},\left \{y(x)\to \exp \left (\frac {-c_1-2 a \text {erf}^{-1}\left (i \sqrt {\frac {2}{\pi }} \sqrt {a e^{\frac {c_1}{a}} \left (c_2+x\right ){}^2}\right ){}^2}{2 a}\right )\right \}\right \} \]
Maple: cpu = 1.669 (sec), leaf count = 55 \[ \left \{ \int ^{y \left ( x \right ) }\!{\frac {1}{\sqrt {2\,a\ln \left ( {\it \_a} \right ) -2\,{\it \_C1}\,a}}}{d{\it \_a}}-x-{\it \_C2 }=0,\int ^{y \left ( x \right ) }\!-{\frac {1}{\sqrt {2\,a\ln \left ( { \it \_a} \right ) -2\,{\it \_C1}\,a}}}{d{\it \_a}}-x-{\it \_C2}=0 \right \} \]