On Littlewood’s boundedness problem for sublinear Duffing equations
On Littlewood’s boundedness problem for sublinear Duffing equations
In this paper, we are concerned with the boundedness of all the solutions and the existence of quasi-periodic solutions for second order differential equations \[ x^{\prime \prime } + g(x) = e(t), \] where the 1-periodic function $e(t)$ is a smooth function and $g(x)$ satisfies sublinearity: \[ \operatorname {sign}(x)\cdot g(x)\to …