\[ \left (-a-x^2\right ) y(x)+4 y''(x)=0 \] ✓ Mathematica : cpu = 0.0066156 (sec), leaf count = 36
\[\left \{\left \{y(x)\to c_1 D_{\frac {1}{4} (-a-2)}(x)+c_2 D_{\frac {a-2}{4}}(i x)\right \}\right \}\] ✓ Maple : cpu = 0.079 (sec), leaf count = 33
\[y \relax (x ) = \frac {c_{2} \WhittakerW \left (-\frac {a}{8}, \frac {1}{4}, \frac {x^{2}}{2}\right )+c_{1} \WhittakerM \left (-\frac {a}{8}, \frac {1}{4}, \frac {x^{2}}{2}\right )}{\sqrt {x}}\]