When a particle of mass m moves on the x-axis in a potential of the form V(x) = kx2, it performs simple harmonic motion. The corresponding time period is proportional to $\sqrt {{m \over k}}$, as can be seen easily using dimensional analysis. However, the motion of a particle can be periodic even when its potential energy increases on both sides of x = 0 in a way different from kx2 and its total energy is such that the particle does not escape to infinity. Consider a particle of mass m moving on the x-axis. Its potential energy is V(x) = $\alpha$ x4 ($\alpha$> 0) for | x | near the origin and becomes a constant equal to V0 for (see figure). The acceleration of this particle for $|x| \gt {X_0}$ is