Let $H$ be a [Hilbert space](/page/Hilbert%20Space), let $e_1,\ldots,e_n$ be orthonormal vectors in $H$, and let $a_1,\ldots,a_n$ be scalars. Then
\begin{align*}
\left\|\sum_{k=1}^n a_k e_k\right\|_H^2=\sum_{k=1}^n|a_k|^2.
\end{align*}
Analysis
Discussion
States the finite Pythagorean identity for linear combinations of orthonormal vectors in an inner product space.