Math: Solving Linear Functionals w/ Riesz Representation

Click For Summary

Discussion Overview

The discussion revolves around solving a problem related to linear functionals and the Riesz representation theorem in the context of R^4 with the usual inner product. Participants explore the definitions and implications of linear functionals, inner products, and specific examples related to these concepts.

Discussion Character

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

Main Points Raised

  • One participant expresses difficulty in connecting the concepts of Riesz representation to a specific problem involving the vector (1,1,2,2).
  • Another participant suggests that the problem is asking for the continuous linear functional associated with the vector in R^4, referencing a resource for clarification.
  • Questions are raised about the definition of the inner product and what the variable x represents in this context.
  • It is noted that the usual inner product on R^4 is the dot product.
  • A participant states that the linear functional associated with a vector v in an inner product space is defined as f(x) = , where < , > denotes the inner product.
  • There is confusion regarding the application of the linear functional and its relation to the original problem, with one participant questioning how the examples provided relate to their specific issue.
  • Another participant clarifies that the linear functional maps into a scalar, which was a key piece of information for understanding the problem.
  • Further discussion leads to the identification of the vector needed to achieve a specific linear functional defined by summing components of a vector in R^n.
  • Participants agree that the vector needed for the linear functional that sums components is the (1,1,...,1) vector.

Areas of Agreement / Disagreement

Participants generally agree on the definition of the linear functional and its relationship to the inner product. However, there remains some confusion and lack of clarity regarding specific applications and interpretations of the concepts discussed, indicating that the discussion is not fully resolved.

Contextual Notes

There are unresolved questions about the definitions and applications of inner products and linear functionals, as well as the specific role of the variable x in the context of the problems being discussed.

sharkboy
Messages
7
Reaction score
0
How do I solve this problem- I know it has something to do Riesz represenation but am having difficulty connecting dots

Conside R4 with usual inner product. Find the linear funcitonal associated to the vector (1,1,2,2).

What am I missing- is this problem complete or is there something else Also what does usual inner product mean.

Sharkie
 
Physics news on Phys.org
Did you read the http://planetmath.org/encyclopedia/RieszRepresentationTheorem.html ? I would suspect this question is asking you to find the continuous linear functional on R^4 (a Hilbert space) associated with u=(1,1,2,2) (u is used as in the notation on the planetmath website).
 
Last edited by a moderator:
So in the inner product what is x?

What is definition of inner product.
 
Wasn't clear in my last post:

there were 2 questions

1- So in the inner product (in planetmath.org) what is x?

2- What is definition of inner product.
 
The usual inner product on R^4 is the dot product.
 
If I remember correctly, the "linear functional" associated with the given vector v in an inner product space is just f(x)= <v, x> where < , > is the inner product.
 
Morphism - Do I just <v,v> for the linear functional. I don't clearly understand what the other term I need.

Sharkie
 
Is f(x) = <(1,2), x> a linear functional on R^2?
 
Yes.. the way to prove is below

Scalar addition

f(u) = <(1,2), u> = 1*u + 2*u = u + 2u = 3u
f(g) = <(1,2), g> = = 3g
f(u+g) = <(1,2), (u,g)> = 1*u + 1*g + 2*u + 2*g = 3u + 3g

f(u) + f(g) = f(u+g)

Scalar multiplication

f(kx) = <(1,2), kx> = kx*1 + 2*kx = 3kx
kf(x) = k <(1,2), x> = k(1*x + 2*x) = 3kx

Since it is closed under both scalar multiplication and additoin, it is a linear functional.

But how does it help me in my actual problem ?
Sorry I don't see the angle

Sharkie
 
  • #10
Your question has been answered several times in these responses! The linear functional associated with vector v is f(x)= <v, x>.

Oh, and since R4 is finite dimensional, talking about "Hilbert Spaces" and "Riesz representation" is overkill!
 
  • #11
OK - I think I was missing a key info and finally figured it out. On more reading, I realized that a linear functional maps into a scalar. Thats the key I was missing. And all the other exampls made sense then. However, this problem is not

Consider the linear functional f:Rn --> R defined by f(x1,x2,. . .,xn)= x1 + x2 +. . . + xn. Find the vector u in Rn such that for all vectors v in Rn we have f(v)=(v,u), where ( , ) is the usual inner product.

The reason it isn't make sense is where does vector x come into the picture.
 
  • #12
They're asking what vector you have to dot with x1,...,xn to get x1+...+xn
 
  • #13
Isn't that just the (1...1) vector (size 1 x n)
 
  • #14
Yeah
 

Similar threads

  • · Replies 2 ·
Replies
2
Views
2K
  • · Replies 8 ·
Replies
8
Views
5K
  • · Replies 43 ·
2
Replies
43
Views
8K
Replies
12
Views
2K
Replies
16
Views
3K
  • · Replies 2 ·
Replies
2
Views
2K
  • · Replies 3 ·
Replies
3
Views
2K
  • · Replies 0 ·
Replies
0
Views
910
  • · Replies 1 ·
Replies
1
Views
3K
  • · Replies 80 ·
3
Replies
80
Views
10K