Harmonics decaying as 1 / n^2

Hi,

I'm reading an article about a class E power amplifier, with an output series resonant circuit, which shows high impedance to harmonics.

It was said there that the harmonic voltages decrease as 1/n^2, which is expected because the amplifier has a continuous collector voltage waveform.

Why is it that having a continuous collector voltage waveform causes a 1/n^2 decrease in harmonic voltages?

Thanks!