Math 2.0

Functional analysis Definition

Strong dual space

Builds on Continuous dual spacevon Neumann bounded setTopology generated by a family of seminorms

Definition

Let 𝕂\mathbb{K} be ℝ\R or ℂ\C, let EE be a topological vector space over 𝕂\mathbb{K}, and let E′E' be its continuous dual space. For each von Neumann bounded set B⊆EB \subseteq E, put pB(f)=sup⁡x∈B∣f(x)∣p_B(f) = \sup_{x \in B} \abs{f(x)} for f∈E′f \in E'. The strong dual space Eb′E'_b of EE is E′E' with the topology generated by the seminorms pBp_B, for all bounded subsets BB of EE.

Remarks

Each pBp_B is finite, because a continuous linear form maps bounded sets to bounded subsets of 𝕂\mathbb{K}. A net (fi)(f_i) converges to ff in Eb′E'_b if and only if fi→ff_i \to f uniformly on every bounded subset of EE. The topology is also written β(E′,E)\beta(E', E). Example: let EE be a normed space with closed unit ball B1B_1. Every bounded set lies in some rB1r B_1, and prB1(f)=r∥f∥p_{r B_1}(f) = r \norm{f}, so the strong dual topology is the topology of the dual norm. Non-example: for an infinite-dimensional Banach space EE, the weak-* topology σ(E′,E)\sigma(E', E) uses only the finite sets BB. It is not the strong topology, since its neighbourhoods of 00 are not norm bounded.

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  1. Continuous dual space concept
  2. Topology generated by a family of seminorms concept
  3. von Neumann bounded set concept
  4. Strong dual space definition

Further reading

  1. Web https://en.wikipedia.org/wiki/Strong_dual_space
About this page
TagJTCKJWTFEN
URIm2://JTCKJWTFEN
Kinddefinition
WritingW0 stub
MSC46A20
DefinesStrong dual space
UsesContinuous dual space, von Neumann bounded set, Topology generated by a family of seminorms
Changed2 November 2026

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