Norm of a linear transformation

In summary, the statement ||T|| = {max|T(x)| : |x|<=1} is equivalent to ||T|| = {max|T(x)| : |x| = 1}. This can be shown by considering the boundedness of T and using the definition of a linear transformation. By linearity, (1/||x||)Tx = T(x/||x||), which leads to the conclusion that the two statements are equivalent.
  • #1
CarmineCortez
33
0

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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  • #2
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.
 
  • #3
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] ?
 
  • #4
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||)
 
  • #5
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.
 
  • #6
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||)
 
  • #7
Exactly, by linearity! So, what can you conclude? Try to think a bit for yourself.
 

1. What is the norm of a linear transformation?

The norm of a linear transformation is a mathematical concept that measures the 'size' or 'magnitude' of a linear transformation. It can be thought of as the distance between the original vector and the transformed vector.

2. How is the norm of a linear transformation calculated?

The norm of a linear transformation is calculated using a specific formula, known as the 'norm formula'. This formula involves taking the square root of the sum of the squares of the components of the transformed vector.

3. What is the significance of the norm of a linear transformation?

The norm of a linear transformation is important because it helps us understand how much a linear transformation distorts or stretches a vector. It is also used in various mathematical applications, such as optimization and solving linear equations.

4. Can the norm of a linear transformation be negative?

No, the norm of a linear transformation is always a positive value. This is because it is calculated using the square root of the sum of squares, which cannot be negative.

5. How does the norm of a linear transformation relate to the concept of a unit vector?

The norm of a linear transformation is closely related to the concept of a unit vector. A unit vector is a vector with a norm of 1. When a linear transformation is applied to a unit vector, the resulting vector will have a norm equal to the norm of the transformation. This means that the norm of a linear transformation can be seen as a measure of how much the transformation changes the length of a vector.

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