Orthogonality of complex exponentials
= Orthogonality of complex exponentials
For <integers> $m$ and $n$,
$$
\int_0^1 e^{2\pi imx}\overline{e^{2\pi inx}}\,dx=
\begin{cases}1,&m=n,\\0,&m\ne n.\end{cases}
$$
This orthogonality extracts matching coefficients when two <Fourier series> are integrated over one period.