\[ y''(x)-f(x) y(x)=0 \] ✗ Mathematica : cpu = 0.133659 (sec), leaf count = 0 , could not solve
DSolve[-(f[x]*y[x]) + Derivative[2][y][x] == 0, y[x], x]
✗ Maple : cpu = 0. (sec), leaf count = 0 , result contains DESol
\[\{y \left (x \right ) = \mathit {DESol}\left (\left \{-\textit {\_Y} \left (x \right ) f \left (x \right )+\frac {d^{2}}{d x^{2}}\textit {\_Y} \left (x \right )\right \}, \left \{\textit {\_Y} \left (x \right )\right \}\right )\}\]