# Topological isomorphism

1. Oct 4, 2008

### dirk_mec1

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

2. Relevant equations
I think this is relevant:

3. The attempt at a solution
A topological isomorphism implies that T and T-1 are bounded and given is that all cauchy sequences in E are convergent.

But if that's the case then by boundedness all convergent sequences are bounded in het norm of F and are thus also Cauchy seqeunces. Am I thinking in the right direction?

2. Oct 4, 2008

### morphism

You have to prove that every Cauchy sequence in F converges. How is what you're saying doing that?

3. Oct 4, 2008

### dirk_mec1

Okay so that's wrong but suppose I have a cauchy sequence in E: $$|x_n-x_m|_E < \epsilon\ , \forall n,m\geq N$$ how can I prove then that F is also Banach?

4. Oct 4, 2008

### morphism

Why are you taking a cauchy sequence in E? You're supposed to prove that F is a Banach space.

5. Oct 5, 2008

### dirk_mec1

Because it is given that E is Banach what implies that every cauchy sequence converges.

6. Oct 5, 2008

### morphism

What I meant is that it makes more sense to start with a cauchy sequence in F rather than in E. Because you want to prove that every cauchy sequence in F converges.

7. Oct 6, 2008

### dirk_mec1

Can someone give me a hint? Because I've started with a cauchy sequence in F but I honestly do not see what to do next.

8. Oct 6, 2008

### HallsofIvy

Staff Emeritus
Let {an} be a sequence in F. What can you say about {T-1 an}?

9. Oct 7, 2008

### dirk_mec1

Let $$\{ a_n \}$$ be a sequence in F then for all $$n,m \geq N$$ we have:

$$|| T^{-1} (a_n-a_m)||_E \leq c\cdot ||a_n-a_m||_F < c \cdot \epsilon$$

So an is Cauchy in F.

But how do I get it to converge in F with limit a?

10. Oct 7, 2008

### morphism

Look, you want to show that F is a Banach space. So you take an arbitrary cauchy sequence in F and show that it converges in F. What do we have to work with here? We know that F is isomorphic to a Banach space. Use that isomorphism.