# Tagged Questions

Functional analysis is the study of infinite-dimensional vector spaces, often with additional structures (inner product, norm, topology), with typical examples given by function spaces. The subject also includes the study of linear and non-linear operators on these spaces, as well as measure, ...

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### Bessel-like inequality

Let $\{e_n\}$ be an orthonormal sequence in an inner product space E. Then I'm trying to show the following inequality: \sum_1^\infty| \langle x, e_n \rangle \langle y, e_n \rangle | \leq ||x||\...
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### States of a Group Ring

Post-Self-Answer Edit: In an answer I THOUGHT I had found sufficient conditions for $f$ to be a state. The new question (for the bounty) was are the conditions necessary? However I have since ...
I have been scouring the internet for answers for some time and would therefore appreciate a reference or a proof since i'm not able to produce one myself. Let $(\mathcal{X},d)$ be a metric space, ...
### Linear transformation $T$ such that for every extension $\overline{T}$, $\|\overline{T}\|>\|T\|$.
Let $E$ and $F$ be normed spaces such that $\dim F < \infty$, $G$ a subspace of $E$ and $T:G\rightarrow F$ a continuous linear map. I know that there exists a continuous linear extension \$\overline{...