What the heck happens to the angular frequency of a quartic potential oscillator at high amplitudes? I completely understand that it follows simple harmonic motion when the amplitude is small, so x is small and therefore we can disregard the quartic term. But at high enough amplitudes, the motion is no longer SHM and subject to "corrections". I would think that it would take longer to complete a cycle because of the added distance...but I'm not sure at all. Is there any intuitive way to think about this? Mathematical approach? I feel dumb as shit.