Let $H$ be an inner product space and let $(e_k)_{k=1}^{\infty}$ be an orthonormal set. Then for all $h \in H$
$$ \sum_{k=1}^{\infty} \langle e_k,h\rangle^2 \leq ||h||^2 $$________________________________________
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Author of the notes: Antonio J. Pan-Collantes
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