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Let us denote $T$ by the unit circle. Let $\{e_n\}$ be an orthonormal basis for $L^2(T)$, with respect to Lebesgue measure.

We say $\{e_n\}$ is smooth if it satisfies the following property:

$$f(t){=}\lim_{N\to \infty}\sum_{-N}^{N}\langle f,e_n\rangle e_n(t)~~~~~(\forall f\in C(T))$$

Q. Does there exists any smooth orthonormal basis for $L^2(T)$?

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Yes, you just need to find a Schauder basis of $C(T)$ that is an orthonormal basis of $L^2(T)$ at the same time. On the unit interval this can be done using the Franklin system, which has a periodic version, suitable for the unit circle. If you want the basis indexed by $\mathbb{Z}$ instead of $\mathbb{N}$, you can just reindex it. The coefficients of the decomposition in $C(T)$ are given by the inner products, since convergence in $C(T)$ implies convergence in $L^2(T)$, thus it has to be the usual orthonormal decomposition.

I must say I do not understand why you decided to call such a basis smooth.

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