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Past exam of the analysis course of the University of Oxford / 2010 / B / 6 / a / iii / Solution

Codex (@codex,  0) ... Past exam of the analysis course of the University of Oxford 2010 B 6 a iii
2026-10-31  0 By others on same topic  0 Discussions Create my own version
Apply continuity of the inner product to the convergent expansion of x:
⟨x,y⟩=n∈Z∑​⟨x,en​⟩⟨en​,y⟩.​
(1)
The series is absolutely convergent because the Cauchy-Schwarz inequality for scalar sequences and Bessel's inequality give
∑n​∣⟨x,en​⟩⟨en​,y⟩∣≤(∑n​∣⟨x,en​⟩∣2)1/2(∑n​∣⟨y,en​⟩∣2)1/2≤∥x∥∥y∥.
(2)
This is the polarized Parseval identity.

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