\[f(x)=\sum_{n=0}^{\infty}\phi_{\lambda_{n}}(x)\int_{a}^{b}f(y)\overline{\phi_{%
\lambda_{n}}(y)}\,\mathrm{d}y=\sum_{n=0}^{\infty}\widehat{f}(\lambda_{n})\phi_%
{\lambda_{n}}(x)\]