Let $H$ be a [Hilbert space](/page/Hilbert%20Space), let $E \subset H$ be an orthonormal set, and let $x \in H$. The set
\begin{align*}
\{e \in E : (x,e)_H \ne 0\}
\end{align*}
is countable.
AnalysisFunctional Analysis
Discussion
Shows that a vector has nonzero coefficients against at most countably many elements of any orthonormal set.
Proof
No proof available for this theorem.
Prerequisites
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Prerequisites Graph
Interactive dependency map showing how this theorem builds on foundational concepts