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In page Stone–Weierstrass theorem:
"The following is an application of this complex version.
- Fourier series: The set of linear combinations of functions en(x) = e2πinx, n ∈ Z is dense in C([0, 1]/{0, 1}), where we identify the endpoints of the interval [0, 1] to obtain a circle. An important consequence of this is that the en are an orthonormal basis of the space L2([0, 1]) of square-integrable functions on [0, 1].[citation needed]