\[ \boxed { {\frac {\rm d}{{\rm d}x}}y \left ( x \right ) -a \left ( {x}^{n}-x \right ) \left ( y \left ( x \right ) \right ) ^{3}- \left ( y \left ( x \right ) \right ) ^{2}=0} \]
Mathematica: cpu = 33.168712 (sec), leaf count = 31 \[ \text {DSolve}\left [-a \left (x^n-x\right ) y(x)^3+y'(x)-y(x)^2=0,y(x),x\right ] \]
Maple: cpu = 1.248 (sec), leaf count = 0 \[ \text {could not solve} \]
Sage: cpu = 0 (sec), leaf count = 0 \[ \text {Maxima was unable to solve this ODE} \]