Rigged Hilbert Space: Algebraic v.s. Continuous Dual Space

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Discussion Overview

The discussion revolves around the definitions and properties of rigged Hilbert spaces, specifically focusing on the distinction between algebraic and continuous dual spaces. Participants explore the implications of these definitions in the context of convergence and topology within the framework of rigged Hilbert spaces.

Discussion Character

  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • Some participants assert that the definitions of rigged Hilbert spaces refer to the continuous dual space, as indicated by the Riesz representation theorem, which identifies H^* with H.
  • There is a question regarding whether continuity is considered with respect to the metric topology induced by the norm from the inner product on H or another topology related to the dense subset mentioned in external sources.
  • One participant notes that the article states the continuous dual is with respect to a finer topology, suggesting this interpretation aligns with their understanding.
  • Another participant references a PhD thesis by Prof. Rafael de la Madrid, suggesting it contains valuable insights into rigged Hilbert spaces, though they mention it may have minor corrections.
  • There is a query about how convergence in sequences of elements of the subspace induces a topology, with a suggestion that it relates to the definition of sequence convergence and neighborhoods.
  • A participant questions whether the notation ||\cdot ||_{l,m,n} represents a norm for all l,m,n in the natural numbers and seeks to understand the relationship of the topology T to the metric topologies induced by these norms.
  • Further exploration is made into the concept of sequential spaces and whether de la Madrid's topology could be defined by open sets related to converging sequences.
  • There is curiosity about whether the maps ||\cdot ||_{l,m,n} are at least seminorms, as suggested by definitions found in external literature.

Areas of Agreement / Disagreement

Participants express differing views on the nature of the dual space and the implications of convergence and topology, indicating that multiple competing views remain without a clear consensus.

Contextual Notes

There are limitations regarding the assumptions made about the topologies involved and the definitions of convergence, which remain unresolved within the discussion.

Rasalhague
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Definitions of a rigged Hilbert space typically talk about the "dual space" of a certain dense subspace of a given Hilbert space H. Do they mean the algebraic or the continuous dual space (continuous wrt the norm topology on H)?
 
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Since they identify H^* with H (by the Riesz representation theorem), they are talking about the continuous dual. That is: the bounded linear functionals from H to \mathbb{C}.
 
Is it continuous with respect to the metric topology induced by the norm from the inner product on H, or continuous with respect to the other topology for the dense subset mentioned in the Wikipedia article?
 
They say in the article that it is the continuous dual wrt the finer topology. This would make sense.
 
Look on the internet for the PhD thesis of Prof. Rafael de la Madrid. Though some small corrections can be made to it, it's the gem for RHS.
 
In equations (4.2) and (4.3), de la Madrid defines a notion of convergence for sequences of elements of the subspace \Phi, which he says induces a topology, T, for \Phi. How does convergence induce a topology? I guess one works backwards somehow from the definition of sequence convergence: a sequence (s_n ) converges to x\in X iff (s_n ) is in every neighborhood residually.

Is ||\cdot ||_{l,m,n} a norm for every l,m,n \in \mathbb{N}, as the notation suggests? If so, what is the relationship of T to the metric topologies induced by these norms? (Do they induce different topologies? Is T perhaps the intersection of these topologies?)

EDIT: Ah, reading Wikipedia: Sequential space and thence the first Franklin article, Spaces in which sequences suffice, specifically condition (b) in section 0, could it be that de la Madrid's topology comes from defining an open set as one for which every sequence converging to a point in the set is eventually/residually in the set?

The definition of a http://planetmath.org/FrechetSpace.html looks interesting too, in particular the determination of a topology by a countable family of seminorms. I wonder if the topology de la Madrid refers to is determined in such a way. Are his maps ||\cdot ||_{l,m,n} at least seminorms?
 
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