Suppose a vibrating string held at both ends can produce standing waves given by sinusoids $f_1$, $f_2$, $f_3, \ldots$ with increasing frequencies.
According to Wikipedia, "in terms of a superposition of sinusoids (e.g. Fourier series), the fundamental frequency is the lowest frequency sinusoidal in the sum."
So, for the waveform $f_2+f_3$, is the fundamental frequency equal to the frequency of $f_2$, or is it fixed by the length of the string, i.e. always equal to the frequency of $f_1$?