# Completeness of R^2 with sup norm

by bugatti79
Tags: completeness, norm
 P: 652 1. The problem statement, all variables and given/known data Given that R is complete, prove that R^2 with the sup norm is complete 2. Relevant equations 3. The attempt at a solution How may I tackle this? Thanks
P: 652
 Quote by bugatti79 1. The problem statement, all variables and given/known data Given that R is complete, prove that R^2 with the sup norm is complete 2. Relevant equations 3. The attempt at a solution How may I tackle this? Thanks
Let ##x_n=(x_n(1), x_n(2))## be an arbitrary Cauchy sequence ##\in R^2, || ||_\infty##

by definition we have ##||x_n-x||_\infty=sup(|x_n(1)-x(1)|,|x_n(2)-x(2)|)## for ##n \in \mathbb{N}##

ie ##||x_n-x||_\infty \to 0## as ##n \to \infty##.

We need to show that

##x_n(1) \to x(1)##.

Since ##x_n## is Cauchy,

##\exists n_0 \in N## s.t. ##||x_n-x||_\infty < \epsilon \forall n \ge n_0##

we have that

##|x_n(1)-x(1)| < \epsilon/2 \forall n_1 \ge n_0## and

##|x_n(2)-x(2)| < \epsilon/2 \forall n_2 \ge n_0##

##\implies |x_n-x| < \epsilon/2 + \epsilon/2 < \epsilon##...........? Not sure if this right or how to complete it?

Thanks

P: 85

## Completeness of R^2 with sup norm

Where did your x come from?
Cauchy means for all r>0, there exists a natural number p such that for all m,n>p, d(x_m,x_n) < r.
In this case, max{|xn(1)-xm(1)|,|xn(2)-xm(2)|}<r for all n,m>p
P: 652
 Quote by Oster Where did your x come from?
I am defining x as an arbitrary sequence in R^2 to start off the proof, am I not?
 P: 85 I thought x_n was your arbitrary Cauchy sequence. You have an x_n and an x in your post.
P: 652
 Quote by Oster Where did your x come from?
 Quote by bugatti79 Let ##x_n=(x_n(1), x_n(2))## be an arbitrary Cauchy sequence ##\in R^2, || ||_\infty## and let x=(x(1),x(2)). ie ##x_n \to x## if its cauchy. by definition we have ##||x_n-x||_\infty=sup(|x_n(1)-x(1)|,|x_n(2)-x(2)|)## for ##n \in \mathbb{N}## ie ##||x_n-x||_\infty \to 0## as ##n \to \infty##. We need to show that ##x_n(1) \to x(1)##. Since ##x_n## is Cauchy, ##\exists n_0 \in N## s.t. ##||x_n-x||_\infty < \epsilon \forall n \ge n_0## we have that ##|x_n(1)-x(1)| < \epsilon/2 \forall n_1 \ge n_0## and ##|x_n(2)-x(2)| < \epsilon/2 \forall n_2 \ge n_0## ##\implies |x_n-x| < \epsilon/2 + \epsilon/2 < \epsilon##...........? Not sure if this right or how to complete it? Thanks
I should have inserted what is highlighted in red above. Does it makes sense now?
 PF Patron Sci Advisor Emeritus P: 8,837 What is x(1) and x(2)? It looks like you're thinking that ##x(1)=\lim_n x_n(1)## and similarly for (2). But what makes you think that these limits exist?
P: 652
 Quote by Fredrik What is x(1) and x(2)? It looks like you're thinking that ##x(1)=\lim_n x_n(1)## and similarly for (2). But what makes you think that these limits exist?
Yes, thats my approach.

Now, that you mention it, I guess I dont know whether they exist but its seems to be a pattern I've seen when writing these proofs. Ie, assume x_n converges to x and continue. That's all I've done here!...
 PF Patron Sci Advisor Emeritus P: 8,837 So you think you can prove that xn→x by assuming that xn→x? If this had made sense, we could prove any statement by just assuming that it's true.
P: 652
 Quote by Fredrik So you think you can prove that xn→x by assuming that xn→x? If this had made sense, we could prove any statement by just assuming that it's true.
but if the sequence is Cauchy then this assumption is valid, right?
 PF Patron Sci Advisor Emeritus P: 8,837 Which sequence? If you mean ##\langle x_n\rangle##, then no, it's not valid, because you're assuming what you're trying to prove. If you mean ##\langle (x_n)_1\rangle## and ##\langle (x_n)_2\rangle##, then my question is, have you proved that these sequences are Cauchy?
P: 652
 Quote by Fredrik Which sequence? If you mean ##\langle x_n\rangle##, then no, it's not valid, because you're assuming what you're trying to prove. If you mean ##\langle (x_n)_1\rangle## and ##\langle (x_n)_2\rangle##, then my question is, have you proved that these sequences are Cauchy?
Let ##x_n## be an arbitrary Cauchy sequence in R

Proof. given ##\epsilon >0##
##\exists n_0 \in \mathbb{N}## s.t. ##\forall n,m > n_0## then
##||x_n-x_m|| < \epsilon##

Suppose x_n converges to L and let ε>0 be given. Then ##\exists n_0 in N## s.t. ##|x_n-L| < \epsilon/2 \forall n \ge n_0##

Let any integer ##m \ge n## be given. since ##m \ge n \ge n_0## then ##|x_m-L| < \epsilon/2##

By the triangle inequality ##|x_n-x_m|=|(x_n-L)+(L-x_m)| \le |x_m -L|+|x_n-L| < \epsilon/2+\epsilon/2 < \epsilon \implies x_n## is Cauchy

If I have correctly proven this is Cauchy, what is the next step?
 PF Patron Sci Advisor Emeritus P: 8,837 If you delete everything before the word "suppose" (in particular the part where you assume what you want to prove), then what we have left is a correct proof of the claim that every convergent sequence in ℝ is Cauchy. But this doesn't seem to be relevant to the problem you're trying to solve. You need to start with an arbitrary sequence in ##\mathbb R^2## that's Cauchy with respect to the ∞-norm, and then use the fact that it's Cauchy with respect to the ∞-norm to learn something else about it, something you can use to prove that it's convergent with respect to the ∞-norm.
 PF Patron HW Helper Thanks P: 6,742 Bugatti79, you need to develop a strategy to work a problem like this. You are trying to show ##R^2## is complete, meaning every Cauchy sequence in ##R^2## converges to a point in ##R^2##. What you have to work with is that ##R## is complete. So you start with a Cauchy sequence ##\{x_n = (x_n(1),x_n(2))\}## like you did. You are trying to find an ##x=(x(1),x(2))\in R^2## such that ##x_n\to x##. If you could show ##x_n(1)## and ##x_n(2)## converged to something, maybe that something would do for ##x##. How might you show they converge? If they do, how could show that their limits could be used for ##x##? You have been working on relevant material recently.
P: 652
 Quote by Fredrik If you delete everything before the word "suppose" (in particular the part where you assume what you want to prove), then what we have left is a correct proof of the claim that every convergent sequence in ℝ is Cauchy. But this doesn't seem to be relevant to the problem you're trying to solve. You need to start with an arbitrary sequence in ##\mathbb R^2## that's Cauchy with respect to the ∞-norm, and then use the fact that it's Cauchy with respect to the ∞-norm to learn something else about it, something you can use to prove that it's convergent with respect to the ∞-norm.
 Quote by LCKurtz Bugatti79, you need to develop a strategy to work a problem like this. You are trying to show ##R^2## is complete, meaning every Cauchy sequence in ##R^2## converges to a point in ##R^2##. What you have to work with is that ##R## is complete. So you start with a Cauchy sequence ##\{x_n = (x_n(1),x_n(2))\}## like you did. You are trying to find an ##x=(x(1),x(2))\in R^2## such that ##x_n\to x##. If you could show ##x_n(1)## and ##x_n(2)## converged to something, maybe that something would do for ##x##. How might you show they converge? If they do, how could show that their limits could be used for ##x##? You have been working on relevant material recently.
Assume an arbitrary sequence ##x_n \to x ## in ##\mathbb{R^2}##. We need to show ##x_n(1) \to x(1)## and ##x_n(2) \to x(2)## in ##\mathbb{R}##
ie, we need to find ##n_0 \in N## s.t ##|x_n(1)-x(1)| < \epsilon \forall n > n_0##

Since we know ##x_n \to x \in R^2, || ||_\infty## we know ##\exists n_0 \in N## s.t

##||x_n-x||_\infty < \epsilon \forall n \ge n_0## ie

##||(x_n(1)-x(1), x_n(2)-x(2))||_\infty < \epsilon##

We have that ##|x_n(1)-x(1)| \le max (|x_n(1)-x(1)|, |x_n(2)-x(2)|) < \epsilon##
This shows ##x_n(1) \to x(1)## as ##n \to \infty##

Similarly

We have that ##|x_n(2)-x(2)| \le max (|x_n(1)-x(1)|, |x_n(2)-x(2)|) < \epsilon##
This shows ##x_n(2) \to x(2)## as ##n \to \infty##

Now, conversely assume ##x_n(1) \to x(1)## and ##x_n(2) \to x(2)##
Need to show that ##x_n \to x in R^2, || ||_\infty##. Need to find
##n_0 \in N## s.t. ##||x_n-x||_\infty \forall n \ge n_0##

but we know that ##x_n(1) \to x(1), \exists n \in N## s.t.

##|x_n(1)-x(1)| < \epsilon/2 \forall n \ge n_1## and
##|x_n(2)-x(2)| < \epsilon/2 \forall n \ge n_2##

Take ##n_0=max(n_1,n_2)##.

Then ##\forall n > n_0## we have ##||x_n-x||_\infty=max(|x_n(1)-x(1)|,|x_n(2)-x(2)|) < \epsilon/2 + \epsilon/2 < \epsilon##

Therefore
##x_n \to x \implies## R^2 with the sup norm is complete...?
It looks like it is similar to the other question I answered in my other post..?
PF Patron
HW Helper
Thanks
P: 6,742
 Quote by LCKurtz Bugatti79, you need to develop a strategy to work a problem like this. You are trying to show ##R^2## is complete, meaning every Cauchy sequence in ##R^2## converges to a point in ##R^2##. What you have to work with is that ##R## is complete. So you start with a Cauchy sequence ##\{x_n = (x_n(1),x_n(2))\}## like you did. You are trying to find an ##x=(x(1),x(2))\in R^2## such that ##x_n\to x##. If you could show ##x_n(1)## and ##x_n(2)## converged to something, maybe that something would do for ##x##. How might you show they converge? If they do, how could show that their limits could be used for ##x##? You have been working on relevant material recently.
 Quote by bugatti79 Assume an arbitrary sequence ##x_n \to x ## in ##\mathbb{R^2}##.
No, no, no. You are trying to show every Cauchy sequence in ##R^2## converges to a point in ##R^2##. You don't start with an arbitrary (non Cauchy) sequence and assume it already converges to some ##x##. You are trying to prove there is such an ##x##. You need to think more about the strategy I outlined and re-quoted above.
 PF Patron Sci Advisor Emeritus P: 8,837 Bugatti79, you need to rethink your entire approach to proofs. Theorems are always implications, i.e. statements of the form ##A\Rightarrow B##, but you always ignore that. You always try to avoid making the assumption A, you always try to avoid using the definitions of the terms in A, and most of the time, you even assume B! These are the three biggest mistakes that can possibly be made in a proof. You also often make irrelevant assumptions that have nothing to do with the theorem. You want to prove that if a sequence is Cauchy with respect to the ∞-norm, it's convergent with respect to the ∞-norm. So A is the statement "##\langle x_n\rangle## is Cauchy with respect to the ∞-norm", and B is the statement "##\langle x_n\rangle## is convergent with respect to the ∞-norm". And you start by assuming B, as usual. This is the single biggest mistake that can be made in a proof. One thing you need to understand is that once a proof of ##A\Rightarrow B## has arrived at the statement B, there's nothing more to say. That's the end of the proof. So if you start by assuming B, nothing more needs to be said. In fact, it wouldn't make any sense to say anything more after that.

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