# Find the linear transformation

1. Mar 3, 2008

### Physicsissuef

1. The problem statement, all variables and given/known data

Check if the linear transformation $$f : \mathbb{R}^2 \rightarrow \mathbb{R}^2$$, defined with $$f(x,y)=(x+y,y)$$ is isomorphism? If so, find the linear transformation $$f^-^1$$

2. Relevant equations

V and U are vector sets. The linear copying $$F:V \rightarrow U$$ which is bijection is isomorphism.

3. The attempt at a solution

2. Mar 3, 2008

### Dick

Linear 'map', not 'copying' right? Pick a general point in the image R^2, say (u,v). Can you solve f(x,y)=(x+y,y)=(u,v) for x and y uniquely in terms of u and v? I think you can. If so then the expression for x and y in terms of u and v is f^(-1) and the map is an isomorphism.

3. Mar 4, 2008

### HallsofIvy

Staff Emeritus
It is also true that if a linear map is a surjection, it is a bijection and an isomorphism. I say that because showing that a linear map is a surjection is essentially the same as finding the inverse function which you are also asked to do.

Suppose (u,v) is any member of R2. You want to show that there exist (x,y) in R2 so that f(x,y)= (u, v). If that is always true, then f is a surjection and so an isomorphism. f(x,y)= (x+y,y)= (u, v) so we have x+ y= u, y= v. Can you solve that for x and y no matter what u and v are? If so then f is an isomorphism. The best way to show that is to actually solve for x= ..., y= .... And once you have done that the function f(u,v)= ( x, y) is the inverse function.

4. Mar 4, 2008

### Physicsissuef

And can you please give me some example, where it wouldn't be bijection, but it will be linear map?

5. Mar 4, 2008

### Dick

f(x,y)=(x,0) is a linear map that's not a surjection (onto). g:R^2->R^1, g(x,y)=x+y is a surjection but not a injection (1-1). Neither is a bijection for two different reasons.

6. Mar 4, 2008

### Physicsissuef

Why f(x,y)=(x,0), it is not surjection?

why g(x,y)=x+y is is a surjection, but not injection?

7. Mar 4, 2008

### Dick

1) Because there isn't any vector (x,y) such that f(x,y)=(0,1). It's not onto.
2) Because g(1,-1)=g(2,-2)=0. It's not 1-1.

8. Mar 4, 2008

### Physicsissuef

We prove surjection for the first f(x,y)=(0,1)=(p,t), so p=0, and t=1, why it is not surjection?

9. Mar 4, 2008

### Dick

No. You don't. You want to find X and Y, such that f(X,Y)=(0,1). f(X,Y)=(X,0). (X,0) can NEVER be equal to (0,1).

10. Mar 4, 2008

### Physicsissuef

Where did you get f(X,Y)=(X,0) from?

11. Mar 4, 2008

### Dick

I DEFINED IT. You asked for an example of a linear map that was not a surjection. I suggested f(x,y)=(x,0). What f have you been talking about? You're not paying attention.

12. Mar 4, 2008

### Physicsissuef

The f(x,y)=x+y is not injection, because of $$(x_1,y_1) \neq (x_2,y_2) , g(x_1,y_1)=g(x_2,y_2)$$, right?

Last edited: Mar 4, 2008
13. Mar 4, 2008

### HallsofIvy

Staff Emeritus
I have no idea what that sentence means! What do you mean by "Do we get arbitrary (0, 1)"?

Physicsissuef, you asked for an example of a linear map that is NOT an isomorphism. Since an isomorphism must have an inverse, it must be both injective and surjective. Since a linear map is surjective if and only if it is injective it is enough to give an example of a linear map that is not injective- that has non-trivial kernel. Surely you have posted enough "kernel" problems here to be familiar with that!

f(x,y)= (x, 0) is obviously not injective because any vector of the form (0, y) is mapped to (0, 0), f(0, y)= (0, 0), and so is in the kernel. So it cannot be an isomorphism and is an example of a linear map that is not an isomorphism.

14. Mar 4, 2008

### Physicsissuef

Wait, wait... Lets clear some things. If some linear map is injective, it is not necessarily surjective or vice versa. What does the kernel has to do now?

15. Mar 4, 2008

### HallsofIvy

Staff Emeritus
What the "kernel" has to do with that is that "injective" means "one to one"- If f:U->V, one and only one member of U is mapped into one member of V. In particular, only one member of U is mapped into 0 (the only thing that must be in every V). Since any linear map must take 0 to 0, that means that only 0 is mapped into 0- a linear map is "injective" if and only if its kernel is the trivial subspace, {0}.

For this problem, or any problem in which "U" and "V" (f:U->V, remember. In this problem U= V= R2) have the same dimension, if f is injective, so the kernel has dimension 0, by the "dimension theorem" I have mentioned before, the image of f must be the same as the dimension of U which is the same as the dimension of V: the image of f is all of V so f is surjective.

If f:U->V with dim(U)< dim(V), since the dimension of the image of f cannot be greater than the dimension of U so f is not surjective. If f:U->V with dim(U)> dim(V), since the dimension of the image of f cannot be greater than the dimension of U, we must have dimension of kernel of f larger than 0 and f is not injective. In either case, f is not an isomorphism which is what we would expect: if two subspaces are "isomorphic" then they are essentially the same- in particular, they have the same dimension.

16. Mar 5, 2008

### Physicsissuef

And how will explain with kernel and dimension with my problem?
$$f : \mathbb{R}^2 \rightarrow \mathbb{R}^2$$

$$f(x,y)=(x+y,y)$$

17. Mar 5, 2008

### Dick

The kernel is zero dimensional. So it's injective. The range is two dimensional. So it's onto. All of these are just fancy ways of saying (x+y,y)=(x'+y',y') only if (x,y)=(x',y') (injective) and for any (x,y), f(x-y,y)=(x,y) (surjective).

18. Mar 5, 2008

### Physicsissuef

f(x,y)= (x, 0)
Here f(x,0)= (x,0)
and f(y,0)= (0,0), right?
Why then it is not injective?
Lets say we will get arbitrary value for x and y two times.
$x_1=1$
$y_1=2$
f(1,0)=(1,0)
f(2,0)=(0,0)
$x_2=1$
$y_2=-1$
f(1,0)=(1,0)
f(-1,0)=(0,0)
So $$f(2,0)=f(-1,0)$$ but $$y_1 \neq y_2$$, right?
Now f:U -> V
U=V=R2
dimU=2
dimV=1
So dim(U)>dim(V), it is also not surjective, right?

Last edited: Mar 5, 2008
19. Mar 5, 2008

### Dick

If f is defined by f(x,y)=(x,0), then f(x,0)=(x,0) and f(y,0)=(y,0). Your problem may be less with linear maps and more with not understanding function notation, because you are posting nonsense.

20. Mar 5, 2008

### Physicsissuef

What was wrong with my explanation?
If f(x,y)=(x,0)
that means that:
f(x,0)=(x,0)
f(0,y)=(0,y)=(0,0)
I still can't undestand what you want to say with the kernel and images. Please tell me, when its/its not surjection, its/its not injection and please give me example.