Inverse of the adjoint of the shift operator

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    Inverse Operator Shift
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Discussion Overview

The discussion revolves around the properties of the shift operator on the Hardy space H^2, specifically focusing on the identity involving the inverse of the adjoint of the shift operator and its application to a function f. Participants explore the implications of this identity and the behavior of the shift operator and its adjoint.

Discussion Character

  • Technical explanation
  • Mathematical reasoning

Main Points Raised

  • One participant defines the shift operator S and poses a question regarding the identity involving (1-\lambda S^*)^{-1} and its application to f.
  • Another participant suggests that understanding what S^* does is crucial, hinting at the importance of an orthonormal basis in H^2.
  • A different participant asserts that S^* can be expressed as (f(z)-f(0))/z, but expresses uncertainty about the term (1-\lambda S^*)^{-1} in the context of the identity.
  • One participant proposes rearranging the equation to show that S^*f(z) can be expressed in terms of (1-\lambda S^*) and the function f.

Areas of Agreement / Disagreement

Participants express differing levels of understanding regarding the adjoint operator S^* and the identity in question. There is no consensus on the best approach to prove the identity, and some uncertainty remains about the implications of the terms involved.

Contextual Notes

Participants have not fully established the properties of the adjoint operator S^*, and there may be missing assumptions regarding the definitions and behavior of the operators involved. The discussion does not resolve the mathematical steps needed to prove the identity.

Likemath2014
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Hi there,

Let [itex]S[/itex] denote the shift operator on the Hardy space on the unit disc [itex]H^2[/itex], that is [itex](Sf)(z)=zf(z)[/itex].

My question is to show the following identity

[itex](1-\lambda S^*)^{-1}S^*f (z)=\frac{f(z)-f(\lambda)}{z-\lambda},[/itex]

where [itex]\lambda,z\in\mathbb{D}[/itex]

Thanks in advance
 
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First of all, can you figure out what ##S^*## does exactly? This can be made easy if you can figure out an orthonormal basis of ##H^2##.
 
Yes, [itex]S^*=\frac{f(z)-f(0)}{z}[/itex]. But my problem is with the term [itex](1-\lambda S^*)^{-1}[/itex].
 
Just put it on the other side. So you need to prove

[tex]S^*f(z) = (1-\lambda S^*)\frac{f(z)-f(\lambda)}{z-\lambda}[/tex]
 

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