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The fundamental frequency of a periodic or quasi-periodic function is the reciprocal of the shortest period of the periodic function.

A function $x(t)$ is periodic with period $P>0$ if $$x(t+P) = x(t) \quad \text{for all } -\infty<t<\infty $$

A function $x(t)$ is quasi-periodic in the vicinity of time $t_0$ if $$x(t+P) \approx x(t) \quad \text{for } t \approx t_0 $$

If $P$ is a period of $x(t)$, so also is $2P$ or any integer multiple of $P$. So there exists a smallest $P>0$ that satisfies the condition above. The reciprocal of that smallest period is the fundamental frequency:

$$ f_0 = \frac{1}{P} $$