\[ \boxed { x{\frac {\rm d}{{\rm d}x}}y \left ( x \right ) +x \left ( y \left ( x \right ) \right ) ^{2}+a=0} \]
Mathematica: cpu = 0.007501 (sec), leaf count = 157 \[ \left \{\left \{y(x)\to -\frac {c_1 J_1\left (2 i \sqrt {-a} \sqrt {x}\right )+i \sqrt {-a} \sqrt {x} \left (c_1 J_0\left (2 i \sqrt {-a} \sqrt {x}\right )-c_1 J_2\left (2 i \sqrt {-a} \sqrt {x}\right )-2 J_0\left (2 i \sqrt {-a} \sqrt {x}\right )\right )}{2 x \left (J_1\left (2 i \sqrt {-a} \sqrt {x}\right )-c_1 J_1\left (2 i \sqrt {-a} \sqrt {x}\right )\right )}\right \}\right \} \]
Maple: cpu = 0.062 (sec), leaf count = 59 \[ \left \{ y \left ( x \right ) ={1\sqrt {a} \left ( {{\sl J}_{0}\left (2\, \sqrt {a}\sqrt {x}\right )}{\it \_C1}+{{\sl Y}_{0}\left (2\,\sqrt {a} \sqrt {x}\right )} \right ) {\frac {1}{\sqrt {x}}} \left ( {\it \_C1}\,{ {\sl J}_{1}\left (2\,\sqrt {a}\sqrt {x}\right )}+{{\sl Y}_{1}\left (2\, \sqrt {a}\sqrt {x}\right )} \right ) ^{-1}} \right \} \]