Homework Help: Prove f is bounded on A using uniform continuity

1. Mar 29, 2012

kingstrick

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

Prove that if F is uniformaly continuous on a bounded subset of ℝ, then F is bounded on A.

2. Relevant equations

3. The attempt at a solution

F is uniformaly continuous on a bounded subset on A in ℝ. Therefore each ε>0, there exists δ(ε)>0 st. if x, u is in A where |x-u|<δ(ε) then |f(x)-f(u)|<ε. Since f is uniformaly continuous on A, then it is continuous at every point of A. Since A is a bounded subset, there exists m>0 st m > |x| for all x in A. Lastly, assume f is unbounded on A. Since f is unbounded on A, if given k >0 there exists a sequence (Xk) in A where |f(Xk)|>k for all k in ℝ. Since A is bdd, seq (Xk) is bdd. Since F is continuous at every point and A is bdd. then there exist a subequence of (Xk) denoted by (Xkn) that converges to x. Then it follows that F(Xkn) converges to F(x). Therefore F(Xkn) must be bounded. This is a contradiction a this would imply that there exists k0 where |F(Xkn)|≤ k0 while |F(Xkn)| should be ≥ k for all k in ℝ.

**My proofs are always long winded like this**
If at all possible, could somebody first, help me if i solved this problem wrong, and if i am right, show me a more concise way to write this problem up.

2. Mar 29, 2012

jgens

Everything through this point is correct.

Here is where you are going to run into difficulties. The sequence $\{x_k\}_{k \in \mathbb{N}}$ converges in $\mathbb{R}$ but that does not mean it converges in $A$. To make this proof work, you would need $A$ to be a closed (and bounded) subset of $\mathbb{R}$.

This part of the argument is contingent upon the fact that $\{x_k\}_{k \in \mathbb{N}}$ converges in $A$. Since $A$ is not closed, this does not necessarily follow.

I cannot think of any way to salvage your proof off the top of my head, but I can suggest how I would prove the result. Choose $0 < \delta$ such that $|f(x_1)-f(x_2)| < 1$ whenever $x_1,x_2 \in A$ satisfy $|x_1-x_2| < \delta$. Since $A$ is bounded, you can cover $A$ by a finite collection of sets with diameter less than $\delta$. Can you show how this forces boundedness?

3. Mar 29, 2012

kingstrick

Is this better?

F is uniformaly continuous on a bounded subset on A in ℝ. Therefore each ε>0, there exists δ(ε)>0 st. if x, u is in A where |x-u|<δ(ε) then |f(x)-f(u)|<ε. Since f is uniformaly continuous on A, then it is continuous at every point of A. Since A is a bounded subset, there exists m>0 st m > |x| for all x in A. Lastly, assume f is unbounded on A. Therefore for all k >0, there exists an x such that |f(x)| ≥k. Therefore for each n in N there is an Xn in A such that |f(Xn)|≥n. The (Xn)'s ae a sequence in the bounded set A. Therefore by the Bolzano-Weirstraus (Xn) has a convergent subsequence (Xnr). So (Xnr) is Cauchy. Thus F is uniforaml continuous at (f(Xnr)). However since |f(Xnr)|≥|f(Xn)|≥n implies (f(Xnr)) is divergent, we have a contradiction.

So F is bounded on A.

4. Mar 29, 2012

jgens

Unfortunately not. It has the same problem as the first argument.

This does not show uniform continuity. Based on your reasoning here, it would probably be a good idea for you to review the definition of uniform continuity.

I will suggest an alternative method of proof that is quite simple: Take $0 < \delta$ such that $|f(x_1)-f(x_2)| < 1$ whenever $x_1,x_2 \in A$ satisfy $|x_1-x_2| < \delta$. Now find a finite cover of $A$ by sets of the form $(x-2^{-1}\delta,x+2^{-1}\delta) \cap A$ (here is where the boundedness of $A$ comes into play). Now use these sets to show that $f$ is necessarily bounded on $A$.

Edit: I thought of a fairly simple way to make your argument by contradiction work. If you really want to go with that method of proof, then I can walk you through that. The idea utilized in the direct proof comes up quite a bit in analysis and properties involving finite covers come up a lot in topology, so I think understanding the direct proof is helpful. If you can't tell, I tend to have a preference for direct proofs rather than proofs by contradiction.

Last edited: Mar 29, 2012
5. Mar 29, 2012

kingstrick

6. Mar 29, 2012

jgens

Suppose that $f$ is uniformly continuous on $A$ but unbounded. For each $k \in \mathbb{N}$ choose $x_k \in A$ such that $k \leq f(x_k)$. The sequence $\{x_k\}_{k \in \mathbb{N}}$ is bounded and therefore has some Cauchy subsequence $\{x_{k_n}\}_{n \in \mathbb{N}}$. Choose $0 < \delta$ such that $|f(x_1)-f(x_2)| < 1$ whenever $x_1,x_2 \in A$ satisfy $|x_1-x_2| < \delta$. Choose $N \in \mathbb{N}$ so large that $N \leq n,m$ implies $|x_{k_n}-x_{k_m}| < \delta$. Can you derive a contradiction from here?

After you get the proof using contradiction, it would really be a good idea to do the direct proof as well.

7. Mar 29, 2012

kingstrick

Since we both derived a subsequence that is cauchy, why doesn't my solution about the cauchy sequences being divergent work as an acceptable contradiction?

8. Mar 30, 2012

jgens

Because you are missing all of the work that shows that this violates uniform continuity. Your level of proof writing is not at the point where you can ask the reader to fill in the details; for one, I am not convinced you actually know why the Cauchy criterion gives you a contradiction. You need to explicitly write out why you get a contradiction.

To my fault, I should have thought about how to use Cauchy sequences to derive a contradiction sooner and pointed you down that road (since you seem to be more comfortable with that).

9. Mar 31, 2012

kingstrick

thank you for your help. I ran out of time.