A ``Different'' Fundamental

Consider a case where the fundamental is not equal to either carrier or modulator frequency:

The ratio of $f_c = 9$ to $f_m = 6$ is 3:2 and the fundamental frequency is given by

$$f_0 = \frac{9}{3} = \frac{6}{2} = 3.$$

Notic the spectrum has only odd harmonics!