\[ \boxed { {\frac {{\rm d}^{n}}{{\rm d}{x}^{n}}}y \left ( x \right ) =f \left ( {\frac {{\rm d}^{n-2}}{{\rm d}{x}^{n-2}}}y \left ( x \right ) \right ) =0} \]
Mathematica: cpu = 0 (sec), leaf count = 1 \[ \text {not solved} \]
Maple: cpu = 0.515 (sec), leaf count = 0 \[ \text {could not solve} \]