# List of Banach spaces

In the mathematical field of functional analysis, Banach spaces are among the most important objects of study. In other areas of mathematical analysis, most spaces which arise in practice turn out to be Banach spaces as well.

## Classical Banach spaces

According to Diestel (1984, Chapter VII), the classical Banach spaces are those defined by Dunford & Schwartz (1958), which is the source for the following table.

Here K denotes the field of real numbers or complex numbers and I is a closed and bounded interval [a,b]. The number p is a real number with 1 < p < ∞, and q is its Hölder conjugate (also with 1 < q < ∞), so that the next equation holds:

${\frac {1}{q}}+{\frac {1}{p}}=1,$

and thus

$q={\frac {p}{p-1}}.$

The symbol Σ denotes a σ-algebra of sets, and Ξ denotes just an algebra of sets (for spaces only requiring finite additivity, such as the ba space). The symbol μ denotes a positive measure: that is, a real-valued positive set function defined on a σ-algebra which is countably additive.

 Dual space Reflexive weakly complete Norm Classical Banach spaces Kn Yes Yes $\|x\|_{2}=\left(\sum _{i=1}^{n}|x_{i}|^{2}\right)^{1/2}$ ℓn1 Yes Yes $\|x\|_{\infty }=\max _{1\leq i\leq n}|x_{i}|$ ℓ1 No No $\|x\|_{\infty }=\sup _{i}|x_{i}|$ ℓ1 No No $\|x\|_{\infty }=\sup _{i}|x_{i}|$ Isomorphic but not isometric to c. ℓ1 + K No Yes $\|x\|_{bv}=|x_{1}|+\sum _{i=1}^{\infty }|x_{i+1}-x_{i}|$ ℓ1 No Yes $\|x\|_{bv_{0}}=\sum _{i=1}^{\infty }|x_{i+1}-x_{i}|$ ba No No $\|x\|_{bs}=\sup _{n}\left|\sum _{i=1}^{n}x_{i}\right|$ Isometrically isomorphic to ℓ∞. ℓ1 No No $\|x\|_{bs}=\sup _{n}\left|\sum _{i=1}^{n}x_{i}\right|$ Isometrically isomorphic to c. rca(X) No No $\|f\|_{B}=\sup _{x\in X}\left|f(x)\right|$ X is a compact Hausdorff space. ? No Yes $\|\mu \|_{ba}=\sup _{A\in \Sigma }|\mu |(A)$ ? No Yes $\|\mu \|_{ba}=\sup _{A\in \Sigma }|\mu |(A)$ ? No Yes $\|\mu \|_{ba}=\sup _{A\in \Sigma }|\mu |(A)$ Lq(μ) Yes Yes $\|f\|_{p}=\left\{\int |f|^{p}\,d\mu \right\}^{1/p}$ 1 < p < ∞ L∞(μ) No ? $\|f\|_{1}=\int |f|\,d\mu$ If the measure μ on S is sigma-finite $N_{\mu }^{\perp }$ No ? $\|f\|_{\infty }\equiv \inf\{C\geq 0:|f(x)|\leq C{\mbox{ for almost every }}x\}.$ where $N_{\mu }^{\perp }=\{\sigma \in ba(\Sigma ):\lambda \ll \mu \}$ ? No Yes $\|f\|_{BV}=\lim _{x\to a^{+}}|f(x)|+V_{f}(I)$ Vf(I) is the total variation of f. ? No Yes $\|f\|_{BV}=V_{f}(I)$ NBV(I) consists of BV functions such that $\lim _{x\to a^{+}}f(x)=0$ . K+L∞(I) No Yes $\|f\|_{BV}=\lim _{x\to a^{+}}|f(x)|+V_{f}(I)$ Isomorphic to the Sobolev space W1,1(I). rca([a,b]) No No $\|f\|=\sum _{i=0}^{n}\sup _{x\in [a,b]}|f^{(i)}(x)|.$ Isomorphic to Rn ⊕ C([a,b]), essentially by Taylor's theorem.

## Banach spaces serving as counterexamples

• James' space, a Banach space that has a Schauder basis, but has no unconditional Schauder Basis. Also, James' space is isometrically isomorphic to its double dual, but fails to be reflexive.
• Tsirelson space, a reflexive Banach space in which neither p nor c0 can be embedded.
• W.T. Gowers construction of a space X that is isomorphic to $X\oplus X\oplus X$  but not $X\oplus X$  serves as a counterexample for weakening the premises of the Schroeder–Bernstein theorem