\[ y''(x)-f(x) y(x)=0 \] ✗ Mathematica : cpu = 0.113838 (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
\[ \left \{ y \left ( x \right ) ={\it DESol} \left ( \left \{ -f \left ( x \right ) {\it \_Y} \left ( x \right ) +{\frac {{\rm d}^{2}}{{\rm d}{x}^{2}}}{\it \_Y} \left ( x \right ) \right \} , \left \{ {\it \_Y} \left ( x \right ) \right \} \right ) \right \} \]