Missing Harmonics in FM Spectrum

For $N_2 > 1$, every $N_2^{\mbox{th}}$ harmonic of $f_0$ is missing in the spectrum.

This can be seen in the plot below where the ratio of the carrier to the modulator is 4:3 and $N_2=3$.

Figure 15: Spectrum of a simple FM instrument, where $f_c = 400$, $f_m = 300$, and $I = 2$.

Notice the fundamental frequency $f_0$ is 100 and every third multiple of $f_0$ is missing from the spectrum.