\[ y(x) y''(x)-a=0 \] ✓ Mathematica : cpu = 0.11843 (sec), leaf count = 115
\[\left \{\left \{y(x)\to \exp \left (\frac {-2 a \text {erf}^{-1}\left (-i \sqrt {\frac {2}{\pi }} \sqrt {a e^{\frac {c_1}{a}} (x+c_2){}^2}\right ){}^2-c_1}{2 a}\right )\right \},\left \{y(x)\to \exp \left (\frac {-2 a \text {erf}^{-1}\left (i \sqrt {\frac {2}{\pi }} \sqrt {a e^{\frac {c_1}{a}} (x+c_2){}^2}\right ){}^2-c_1}{2 a}\right )\right \}\right \}\] ✓ Maple : cpu = 0.963 (sec), leaf count = 54
\[\int _{}^{y \relax (x )}\frac {1}{\sqrt {2 \ln \left (\textit {\_a} \right ) a -2 c_{1} a}}d \textit {\_a} -x -c_{2} = 0\]