\[ y^{(3)}(x)-a^2 \left (y'(x)^5+2 y'(x)^3+y'(x)\right )=0 \] ✗ Mathematica : cpu = 10.9462 (sec), leaf count = 0 , could not solve
DSolve[-(a^2*(Derivative[1][y][x] + 2*Derivative[1][y][x]^3 + Derivative[1][y][x]^5)) + Derivative[3][y][x] == 0, y[x], x]
✓ Maple : cpu = 0.378 (sec), leaf count = 95
\[ \left \{ y \left ( x \right ) =\int \!{\it RootOf} \left ( -3\,\int ^{{\it \_Z}}\!{\frac {1}{\sqrt {3\,{a}^{2}{{\it \_f}}^{6}+9\,{{\it \_f}}^{4}{a}^{2}+9\,{a}^{2}{{\it \_f}}^{2}+9\,{\it \_C1}}}}{d{\it \_f}}+x+{\it \_C2} \right ) \,{\rm d}x+{\it \_C3},y \left ( x \right ) =\int \!{\it RootOf} \left ( 3\,\int ^{{\it \_Z}}\!{\frac {1}{\sqrt {3\,{a}^{2}{{\it \_f}}^{6}+9\,{{\it \_f}}^{4}{a}^{2}+9\,{a}^{2}{{\it \_f}}^{2}+9\,{\it \_C1}}}}{d{\it \_f}}+x+{\it \_C2} \right ) \,{\rm d}x+{\it \_C3} \right \} \]