Boundedness of Solutions of a Quasi-periodic Sublinear Duffing Equation

The authors study the Lagrangian stability for the sublinear Duffing equations ẍ + e(t)∣x∣α−1x = p(t) with 0 < α < 1, where e and p are real analytic quasi-periodic functions with frequency ω. It is proved that if the mean value of e is positive and the frequency ω satisfies Diophantine condition, then every solution of the equation is bounded.