Hilbert Manifolds: An Infinite Dimensional Analogy to Smooth Manifolds?

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Discussion Overview

The discussion revolves around the concept of defining a space that is locally homeomorphic to an infinite dimensional Hilbert space, drawing an analogy to the definition of smooth manifolds. Participants explore the implications, applications, and theoretical background of Hilbert manifolds and their geometric structures.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant asks if a space can be defined as locally homeomorphic to an infinite dimensional Hilbert space, suggesting that the definition could mirror that of smooth manifolds.
  • Another participant acknowledges that "locally homeomorphic to a Hilbert space" is a well-defined condition but questions the interest in such objects, noting that it may overlook the algebraic structure of Hilbert space.
  • A different participant introduces an application in machine learning, discussing the use of similarity measures (kernels) that create metrics in Hilbert spaces and speculating on the potential for more complex geometric structures arising from this approach.
  • One participant raises a question about the isometry of separable Hilbert spaces to l2, seeking clarification on this point.
  • Another participant references the historical context of Hilbert manifolds and infinite dimensional Morse theory, citing works by Richard Palais and Steven Smale to emphasize the established nature of the theory.

Areas of Agreement / Disagreement

Participants express varying levels of interest and understanding regarding the definition and implications of Hilbert manifolds. There is no consensus on the significance or applications of these spaces, and questions remain about their geometric structures and algebraic properties.

Contextual Notes

Some assumptions about the relevance of algebraic structure versus geometric structure in the context of Hilbert spaces are not fully explored. The discussion also touches on historical references that may not be universally familiar to all participants.

Who May Find This Useful

Researchers and students in mathematics, particularly those interested in topology, functional analysis, and applications in machine learning, may find this discussion relevant.

Cincinnatus
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Can you define a space that is locally homeomorphic to an infinite dimensional Hilbert space analogously to how you define a (smooth) manifold by an atlas defining local homeomorphisms to R^n?

So the charts would just map to the Hilbert space rather than R^n. Could the rest of the definition be left the same?
 
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"Locally homeomorphic to a Hilbert space" is a perfectly well-defined condition. Such objects might not be interesting to study -- e.g. it completely ignores the algebraic structure of Hilbert space. (compare: in the definition of a manifold, R^n is used for its simple geometry, and not for its vector space structure) What is the the intended application?
 
In machine learning, people are interested in taking sets of documents, images or whatever and defining a similarity measure on them (kernel) which gives rise to a metric in the Hilbert space represented by that kernel. I was wondering if the space you get in this way couldn't have more complicated geometric structure.
 
Someone correct me if I'm rusty, but aren't all separable Hilbert spaces isometric to l2?
 
the theory of hilbert manifolds and infinite dimensional morse theory is mroe than 40 years old. see richard palais, morse theory on hilbert manifolds, topology 1963, and a 1964 article in BAMS by steven smale.
 

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