Linked from
The 98 pages that link to Fourier series, each with the reason it gives.
Basel problemRelated: The Fourier series of x² gives a direct route to the sum.
Sine and cosineRelated: The functions form the basic components for decomposing periodic patterns.
Peter–Weyl theoremRelated: On the circle group, Peter–Weyl becomes the familiar Fourier expansion.
Vector-valued functionRelated: Its componentwise form can represent periodic vector-valued signals.