Do Hilbert Space Isomorphism Map Dense Sets to Dense Sets?

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SUMMARY

The discussion confirms that if A : H -> K is a bounded linear operator and an isomorphism between Hilbert spaces H and K, then the image of a dense set X in H, denoted as A(X), is also dense in K. This conclusion is derived from the properties of bounded operators and the definition of dense sets. The argument utilizes the concept of open sets and the contradiction that arises if A(X) were not dense, reinforcing the established relationship between dense sets under isomorphic mappings.

PREREQUISITES
  • Understanding of Hilbert spaces
  • Knowledge of bounded linear operators
  • Familiarity with the concept of dense sets in topology
  • Basic principles of isomorphism in functional analysis
NEXT STEPS
  • Study the properties of bounded linear operators in functional analysis
  • Explore the concept of dense sets in various topological spaces
  • Learn about isomorphisms in Hilbert spaces and their implications
  • Review texts on functional analysis, such as "Functional Analysis" by Walter Rudin
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Mathematicians, students of functional analysis, and researchers interested in the properties of Hilbert spaces and linear operators.

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Suppose that H, K are Hilbert spaces, and A : H -> K is a bounded linear operator and an isomorphism.

If X is a dense set in H, then is A(X) a dense set in K?

Any references to texts would also be helpful.
 
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It is true. Suppose, that A(X) is not dense. Then let V be a non-empty open set in K \ A(X). The pullback of V by A is open (A is bounded) and not empty (A is isomorphism), and by definition it is not in X, which contradicts the fact, that X is dense in H.
 
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