\[\sideset{}{{}^{\prime\prime}}{\sum}_{\ell=0}^{n}T_{j}\left(x_{\ell}\right)T_{k%
}\left(x_{\ell}\right)=\begin{cases}n,&j=k=0\text{ or }n,\\
\frac{1}{2}n,&j=k\neq 0\text{ or }n,\\
0,&j\neq k,\end{cases}\]