= Fourier harmonic
{c}
A sinusoidal component at an integer multiple of a fundamental angular frequency. For a $2\pi$-periodic angular coordinate $\phi$, real <Fourier series> use $\cos(k\phi)$ and $\sin(k\phi)$. Summing over $q$ equally spaced phases removes every component whose integer $k$ is not a multiple of $q$, since $\sum_{j=0}^{q-1}e^{2\pi i k j/q}=0$ in that case. Indeed, the <geometric series> has ratio $w=e^{2\pi i k/q}\ne1$ and sum $(1-w^q)/(1-w)=0$. When $q$ divides $k$, all $q$ terms are one. This cancellation isolates higher-order feedback in <resonant conjunction geometry>.
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