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