How does the map \Phi define an isomorphism between V and V**?

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In summary, the theorem tells us that an isomorphism between two vector spaces is a function that takes an input in one space and outputs an input in the other space.
  • #1
latentcorpse
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I cannot at all understand the theorem on p16 of the notes attached in this thread:Surely seeing as we want an isomoprhism between V and V**, [itex]\Phi[/itex] should act on an element of V i.e. a vector X and take it to an element of V** (i don't know what such an element would look like though!). But anyway I am immediately lost because the map seems to act not only on a vector but also on a covector [itex]\omega[/itex] which won't be in V.

There is some blurb at the beginning of Wald on this as well but to be honest I think it was, if possible, even less helpful!

An how does it define an isomorphism?

Aaaargh!
 
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  • #2
latentcorpse said:
I cannot at all understand the theorem on p16 of the notes attached in this thread:
In Harvey
Which notes? Can you be more specific? Link?
 
  • #3
arkajad said:
In Harvey
Which notes? Can you be more specific? Link?

Hey. Sorry about that!
The notes are attached in this thread here:
https://www.physicsforums.com/showthread.php?p=3042019#post3042019

For some reason, PF won't let me attach the same thing ni multiple threads which is a bit annoying!
 
  • #4
I do not see any theorem on p. 16.
 
  • #5
arkajad said:
I do not see any theorem on p. 16.

p18. sorry!
 
  • #6
You want to construct the isomorphism [itex]\Phi[/itex] from V to V**. So you want to associate to each vector X in V an element [itex]\Phi(X)[/itex] in V**. But V** is the dual of V*. That is a linear functional on V*. To define a linear functional on V* you say how it acts on elements of V*. So, you chose [itex]\omega[/itex] in V* and you want to define a number that [itex]\Phi(X)[/itex] will associate to [itex]\omega[/itex]. You can write it as follows:

[tex]\Phi(X):\omega \mapsto \omega(X)[/tex]

This is the definition of [itex]\Phi(X)[/itex]. [itex]\Phi(X)[/tex] by definition associates with each [itex]\omega[/itex] it's value on [tex]X[/tex]. Makes sense? And you are supposed to check, writing it down on paper and understanding (so simple that may be difficult!) why this is really a number that linearly depends on [itex]\omega[/itex]. Another way of writing the same is

[tex]\Phi(X)(\omega)=\omega(X)[/tex]

or

[tex](\Phi(X))(\omega)=\omega(X)[/tex]
 
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  • #7
arkajad said:
You want to construct the isomorphism [itex]\Phi[/itex] from V to V**. So you want to associate to each vector X in V an element [itex]\Phi(X)[/itex] in V**. But V** is the dual of V*. That is a linear functional on V*. To define a linear functional on V* you say how it acts on elements of V*. So, you chose [itex]\omega[/itex] in V* and you want to define a number that [itex]\Phi(X)[/itex] will associate to [itex]\omega[/itex]. You can write it as follows:

[tex]\Phi(X):\omega \mapsto \omega(X)[/tex]

This is the definition of [itex]\Phi(X)[/itex]. [itex]\Phi(X)[/tex] by definition associates with each [itex]\omega[/itex] it's value on [tex]X[/tex]. Makes sense? And you are supposed to check, writing it down on paper and understanding (so simple that may be difficult!) why this is really a number that linearly depends on [itex]\omega[/itex]. Another way of writing the same is

[tex]\Phi(X)(\omega)=\omega(X)[/tex]

or

[tex](\Phi(X))(\omega)=\omega(X)[/tex]

But since [itex]\Phi[/itex] is an isomorphism between V and V**, shouldn't it act on elements in V? I can kind of see that it does since you've written [itex]\Phi(X)[/itex] but it seems like you've got it acting on [itex]\omega[/itex] when you write [itex]\Phi(X) : \omega \mapsto \Phi(X)(\omega)[/itex], no?

Also, when we think of V** as the space of linear functions from V* to R, is our isomorphism basically saying that to each X in V, we associate [itex]\Phi(X)(\omega)=\omega(X)[/itex] which is a linear function from V* to R i.e. an element of V**?

Thanks!
 
  • #8
latentcorpse said:
But since [itex]\Phi[/itex] is an isomorphism between V and V**, shouldn't it act on elements in V? I can kind of see that it does since you've written [itex]\Phi(X)[/itex] but it seems like you've got it acting on [itex]\omega[/itex] when you write [itex]\Phi(X) : \omega \mapsto \Phi(X)(\omega)[/itex], no?

Map from V to V**. [itex]\Phi[/itex] is the map. [itex]X[/itex] is in V. So [itex]\Phi(X)[/itex] must be in V**. How is ane element of V** defined? By saying how it acts on V*. So to define [itex]\Phi(X)[/itex] we must tell what it does with elements of V*. This is what we do.

The story has some similarity with what you can do with functions. Say, you play with functions on set A. You know what are functions, yes? But do you know that each point a of A defines a function on the set of all functions? Think about this:

[tex]a(f)=f(a)[/tex]

The above is the definition. Very precise one.
In the above a is constant, f is the variable.

Contemplate it for a while and you will understand how tricky and clever the math notation can be.
 
  • #9
There is another way of doing the same. For [itex]X\in V[/tex] let us denote by [itex]X^{**}[/itex] the element of V** that we want to associate with X. We define X** as follows

[tex]X^{**}(\omega)=\omega(X)[/tex]

Is X** well defined? Is it indeed an element of V**? Yes? If so, we will denote the map [itex]X\mapsto X^{**}[/itex] by [itex]\Phi[/itex]. Better?
 

FAQ: How does the map \Phi define an isomorphism between V and V**?

What are tensors and isomorphisms?

Tensors are mathematical objects that describe the geometric properties of a space. They are used to represent linear transformations between vector spaces. Isomorphisms, on the other hand, are mappings between two mathematical structures that preserve the underlying structure.

What is the difference between tensors and scalars?

Tensors are a generalization of scalars, which are single numbers with no direction or orientation. Tensors, on the other hand, have direction and orientation and can be represented as multi-dimensional arrays of numbers.

How are tensors and isomorphisms used in physics?

In physics, tensors are used to describe physical quantities, such as force, velocity, and stress. Isomorphisms are used to map between different coordinate systems, allowing us to describe physical phenomena in different frames of reference.

What are some examples of tensors and isomorphisms?

Examples of tensors include the stress tensor, which describes the forces acting on a material, and the electromagnetic tensor, which describes the electromagnetic field. Isomorphisms can include rotations and reflections in geometry, and unit conversions in physics.

How are tensors and isomorphisms related to each other?

Tensors and isomorphisms are closely related in that they both involve transformations between mathematical structures. Tensors can be thought of as a type of isomorphism, as they map between vector spaces. Isomorphisms can also be used to define tensor properties, such as symmetry and skew-symmetry.

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