A function representable as a generalized Fourier series. Let R be a metric space with metric ρ(x, y). Following Bohr, a continuous function x(t) for (-∞0, there exists ℓ = ℓ(ϵ)>0 such that every interval [t_0, t_0 + ℓ(ϵ)] contains at least one number τ for which ρ[x(t), x(t + τ)]<ϵ for (-∞

Fourier series | periodic function