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18 changes: 13 additions & 5 deletions spaces/S000030/README.md
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---
uid: S000030
name: Hilbert space
name: Hilbert space $\ell^2$
aliases:
- $\ell^2$
- Fréchet space
- Fréchet space $\mathbb R^\omega$
- Continuous real-valued functions on the unit interval
- $C([0,1])$
counterexamples_id: 36
refs:
- doi: 10.1007/978-1-4612-6290-9
Expand All @@ -17,5 +18,12 @@ Let $X$ be the space $\ell^2$ of all sequences $x = \langle x_i \rangle$ of real

This space is homeomorphic to $\mathbb{R}^\omega$ with the product topology (Fréchet space). For this deep result due to R. D. Anderson, see for example {{doi:10.1090/S0002-9904-1968-12044-0}}.

Defined as counterexamples #36 ("Hilbert Space") and #37 ("Fréchet Space")
in {{doi:10.1007/978-1-4612-6290-9}}.
More generally, any infinite-dimensional separable Banach space or separable Fréchet space is homeomorphic to $\mathbb R^\omega$. See {{wikipedia:Anderson–Kadec theorem}}.

In particular, the following are all homeomorphic as topological spaces:
- $\mathbb R^\omega$
- Sequence spaces $\ell^p$ ($1 \leq p < + \infty$)
- Spaces $L^p([0,1])$ ($1 \leq p < + \infty$) of Lebesgue measurable functions with finite $L^p$-norm
- the Banach space $C([0,1])$ of real-valued continuous functions on $[0,1]$ with the supremum norm

Specific instances of the above are counterexamples #36 ("Hilbert Space"), #37 ("Fréchet Space"), and #108 ("$C[0,1]$") in {{doi:10.1007/978-1-4612-6290-9}}.
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