Norm of a linear transformation

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Homework Help Overview

The discussion revolves around the norm of a linear transformation, specifically the equivalence of two definitions of the norm: one involving the maximum of |T(x)| for |x| ≤ 1 and the other for |x| = 1. Participants are exploring the properties of bounded linear transformations in the context of this equivalence.

Discussion Character

  • Exploratory, Conceptual clarification, Mathematical reasoning

Approaches and Questions Raised

  • Participants discuss the relationship between the vectors T(x) and T(x/||x||), questioning the implications of boundedness and the definitions of the norm. There is an attempt to show equality between the two definitions of the norm.

Discussion Status

Some participants have provided insights into the properties of bounded linear transformations and the implications of linearity. There is an ongoing exploration of the relationship between the vectors involved, with no explicit consensus reached yet.

Contextual Notes

There is uncertainty regarding whether T is a bounded linear transformation, and some participants express this ambiguity. The discussion also touches on the implications of the definitions provided and the assumptions surrounding them.

CarmineCortez
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Homework Statement


||T|| = {max|T(x)| : |x|<=1} show this is equivalent to ||T|| = {max|T(x)| : |x| = 1}



The Attempt at a Solution



{max |T(x)| : x<=1} = {max ||x|| ||T(x/||x||)|| : |x|<=1} <= {max ||T(x)|| : |x| = 1}

does that look right? I need to show equality...
 
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I assume you are speaking of a bounded linear transformation?

If T is bounded, then there exists some constant C so that ||Tx||<=C||x|| for all x from the domain of T, and it follows almost directly from this definition.
 
You're on the right track. What is the relation between the vector [tex]Tx[/tex] and the vector [tex]T\left(\frac{x}{\|x\|}\right)[/tex] ?
 
radou said:
I assume you are speaking of a bounded linear transformation?

If T is bounded, then there exists some constant C so that ||Tx||<=C||x|| for all x from the domain of T, and it follows almost directly from this definition.

I don't know that T is bounded...T is on R^n


Tx >= T(x/||x||)
 
CarmineCortez said:
Tx >= T(x/||x||)

[tex]Tx[/tex] and [tex]T\left(\frac{x}{\|x\|}\right)[/tex] are vectors in the range space of [tex]T[/tex]; they do not possesses an order relation. The relationship between these two vectors is simpler, and follows directly from the definition of a linear transformation.
 
ystael said:
[tex]Tx[/tex] and [tex]T\left(\frac{x}{\|x\|}\right)[/tex] are vectors in the range space of [tex]T[/tex]; they do not possesses an order relation. The relationship between these two vectors is simpler, and follows directly from the definition of a linear transformation.


(1/||x|| ) Tx = T(x/||x||)
 
Exactly, by linearity! So, what can you conclude? Try to think a bit for yourself.
 

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