Proving the statement “$l^1$ is complete.” differently.
$begingroup$
I want to prove $l^1$ is complete. I want to prove that any Cauchy sequence $(x_n)$ is Convergent to $(y)$ whose $y_i$ is the limit of the sequence $x_{(n,i)}$ . I want to prove this part first then I would show that $||y||$ is bounded not the other way around.
Can anyone please help me to proceed? I am really having
trouble to visualize that the Cauchy sequence $(x_n)$ converges to $(y)$.
metric-spaces complete-spaces
$endgroup$
add a comment |
$begingroup$
I want to prove $l^1$ is complete. I want to prove that any Cauchy sequence $(x_n)$ is Convergent to $(y)$ whose $y_i$ is the limit of the sequence $x_{(n,i)}$ . I want to prove this part first then I would show that $||y||$ is bounded not the other way around.
Can anyone please help me to proceed? I am really having
trouble to visualize that the Cauchy sequence $(x_n)$ converges to $(y)$.
metric-spaces complete-spaces
$endgroup$
$begingroup$
if $(x_n)$ is Cauchy, then so are the coordinate sequences $(x^{(j)}_n)$. So if your scalar field is complete, you get a candidate for the limit sequence.
$endgroup$
– Matematleta
Jan 6 at 19:01
$begingroup$
Yea I understood up to the limit point convergence. I have written this in my question. $(y)$ is that sequence. But I have to prove that $(x_n)$ converges to $(y)$.@Matematleta
$endgroup$
– cmi
Jan 6 at 19:04
$begingroup$
Then, I am not sure what you are asking but I have posted a sketch of a proof. If it is not what you are looking for, let me know and I will be happy to delete it.
$endgroup$
– Matematleta
Jan 6 at 19:53
$begingroup$
Please show me how the Cauchy sequence $(x_n)$ converges to $y$.@Matematleta
$endgroup$
– cmi
Jan 6 at 20:01
add a comment |
$begingroup$
I want to prove $l^1$ is complete. I want to prove that any Cauchy sequence $(x_n)$ is Convergent to $(y)$ whose $y_i$ is the limit of the sequence $x_{(n,i)}$ . I want to prove this part first then I would show that $||y||$ is bounded not the other way around.
Can anyone please help me to proceed? I am really having
trouble to visualize that the Cauchy sequence $(x_n)$ converges to $(y)$.
metric-spaces complete-spaces
$endgroup$
I want to prove $l^1$ is complete. I want to prove that any Cauchy sequence $(x_n)$ is Convergent to $(y)$ whose $y_i$ is the limit of the sequence $x_{(n,i)}$ . I want to prove this part first then I would show that $||y||$ is bounded not the other way around.
Can anyone please help me to proceed? I am really having
trouble to visualize that the Cauchy sequence $(x_n)$ converges to $(y)$.
metric-spaces complete-spaces
metric-spaces complete-spaces
asked Jan 6 at 18:51
cmicmi
1,136312
1,136312
$begingroup$
if $(x_n)$ is Cauchy, then so are the coordinate sequences $(x^{(j)}_n)$. So if your scalar field is complete, you get a candidate for the limit sequence.
$endgroup$
– Matematleta
Jan 6 at 19:01
$begingroup$
Yea I understood up to the limit point convergence. I have written this in my question. $(y)$ is that sequence. But I have to prove that $(x_n)$ converges to $(y)$.@Matematleta
$endgroup$
– cmi
Jan 6 at 19:04
$begingroup$
Then, I am not sure what you are asking but I have posted a sketch of a proof. If it is not what you are looking for, let me know and I will be happy to delete it.
$endgroup$
– Matematleta
Jan 6 at 19:53
$begingroup$
Please show me how the Cauchy sequence $(x_n)$ converges to $y$.@Matematleta
$endgroup$
– cmi
Jan 6 at 20:01
add a comment |
$begingroup$
if $(x_n)$ is Cauchy, then so are the coordinate sequences $(x^{(j)}_n)$. So if your scalar field is complete, you get a candidate for the limit sequence.
$endgroup$
– Matematleta
Jan 6 at 19:01
$begingroup$
Yea I understood up to the limit point convergence. I have written this in my question. $(y)$ is that sequence. But I have to prove that $(x_n)$ converges to $(y)$.@Matematleta
$endgroup$
– cmi
Jan 6 at 19:04
$begingroup$
Then, I am not sure what you are asking but I have posted a sketch of a proof. If it is not what you are looking for, let me know and I will be happy to delete it.
$endgroup$
– Matematleta
Jan 6 at 19:53
$begingroup$
Please show me how the Cauchy sequence $(x_n)$ converges to $y$.@Matematleta
$endgroup$
– cmi
Jan 6 at 20:01
$begingroup$
if $(x_n)$ is Cauchy, then so are the coordinate sequences $(x^{(j)}_n)$. So if your scalar field is complete, you get a candidate for the limit sequence.
$endgroup$
– Matematleta
Jan 6 at 19:01
$begingroup$
if $(x_n)$ is Cauchy, then so are the coordinate sequences $(x^{(j)}_n)$. So if your scalar field is complete, you get a candidate for the limit sequence.
$endgroup$
– Matematleta
Jan 6 at 19:01
$begingroup$
Yea I understood up to the limit point convergence. I have written this in my question. $(y)$ is that sequence. But I have to prove that $(x_n)$ converges to $(y)$.@Matematleta
$endgroup$
– cmi
Jan 6 at 19:04
$begingroup$
Yea I understood up to the limit point convergence. I have written this in my question. $(y)$ is that sequence. But I have to prove that $(x_n)$ converges to $(y)$.@Matematleta
$endgroup$
– cmi
Jan 6 at 19:04
$begingroup$
Then, I am not sure what you are asking but I have posted a sketch of a proof. If it is not what you are looking for, let me know and I will be happy to delete it.
$endgroup$
– Matematleta
Jan 6 at 19:53
$begingroup$
Then, I am not sure what you are asking but I have posted a sketch of a proof. If it is not what you are looking for, let me know and I will be happy to delete it.
$endgroup$
– Matematleta
Jan 6 at 19:53
$begingroup$
Please show me how the Cauchy sequence $(x_n)$ converges to $y$.@Matematleta
$endgroup$
– cmi
Jan 6 at 20:01
$begingroup$
Please show me how the Cauchy sequence $(x_n)$ converges to $y$.@Matematleta
$endgroup$
– cmi
Jan 6 at 20:01
add a comment |
1 Answer
1
active
oldest
votes
$begingroup$
Sketch: picking up from our comments, we have $x^{(j)}_nto y_j$ for each $jin mathbb N$.
Step $1$: show that $y=(y_j)in ell^1:$
$|y|:=lim_{Jto infty}sum^J_{j=1}|y_j|=lim_{Jto infty}(sum^J_{j=1}lim_{nto infty}|x^{(j)}_n|)=lim_{Jto infty}lim_{nto infty}(sum^J_{j=1}|x^{(j)}_n|).$
Now, $sum^J_{j=1}|x^{(j)}_n|le sum^{infty}_{j=1}|x^{(j)}_n|=|x_n|le M<infty $ because the original sequence $(x_n)$ is Cauchy, hence bounded. So,
$|y|le lim_{nto infty}M=M.$
Step $2:$ Prove that $|x_n-y|to 0:$
Let $epsilon>0$ and choose an integer $N$ such that $l,n>NRightarrow |x_l-x_n|<epsilon.$
Then, $sum^J_{j=1}|x_n^{(j)}-x_l^{(j)}|le sum^{infty}_{j=1}|x_n^{(j)}-x_l^{(j)}|=|x_n-x_l|<epsilon.$
I'll leave the rest to you: keep $n>N$, fix $J$ and let $lto infty.$ Finally, let $Jto infty.$
$endgroup$
$begingroup$
I have problem in the second part. Please complete it. @Matematleta
$endgroup$
– cmi
Jan 6 at 20:00
$begingroup$
And I have seen this proof earlier. It is kind of escaping proof. It can not be visualized.@Matematleta
$endgroup$
– cmi
Jan 6 at 20:05
$begingroup$
The Cauchy criterion? What? All I am doing is proving that the logical candidate for the limit of the $(x_n)$ is indeed the limit. To do this, you need to show two things 1). it's in $ell^1$. and 2) the sequence $(x_n)$ actually converges to it. I think my sketch is a good roadmap. Suggestion: write the sequence $(x_n)$ in matrix form to see what is going on. The fact that $(x_n)$ is Cauchy in each coordinate is really all you need to know.
$endgroup$
– Matematleta
Jan 6 at 20:06
$begingroup$
without using limit can you show it ?@Matematleta
$endgroup$
– cmi
Jan 6 at 20:43
1
$begingroup$
Your answer helped me to understand what I wanted to understand.@Matematleta
$endgroup$
– cmi
Jan 9 at 4:14
|
show 1 more comment
Your Answer
StackExchange.ifUsing("editor", function () {
return StackExchange.using("mathjaxEditing", function () {
StackExchange.MarkdownEditor.creationCallbacks.add(function (editor, postfix) {
StackExchange.mathjaxEditing.prepareWmdForMathJax(editor, postfix, [["$", "$"], ["\\(","\\)"]]);
});
});
}, "mathjax-editing");
StackExchange.ready(function() {
var channelOptions = {
tags: "".split(" "),
id: "69"
};
initTagRenderer("".split(" "), "".split(" "), channelOptions);
StackExchange.using("externalEditor", function() {
// Have to fire editor after snippets, if snippets enabled
if (StackExchange.settings.snippets.snippetsEnabled) {
StackExchange.using("snippets", function() {
createEditor();
});
}
else {
createEditor();
}
});
function createEditor() {
StackExchange.prepareEditor({
heartbeatType: 'answer',
autoActivateHeartbeat: false,
convertImagesToLinks: true,
noModals: true,
showLowRepImageUploadWarning: true,
reputationToPostImages: 10,
bindNavPrevention: true,
postfix: "",
imageUploader: {
brandingHtml: "Powered by u003ca class="icon-imgur-white" href="https://imgur.com/"u003eu003c/au003e",
contentPolicyHtml: "User contributions licensed under u003ca href="https://creativecommons.org/licenses/by-sa/3.0/"u003ecc by-sa 3.0 with attribution requiredu003c/au003e u003ca href="https://stackoverflow.com/legal/content-policy"u003e(content policy)u003c/au003e",
allowUrls: true
},
noCode: true, onDemand: true,
discardSelector: ".discard-answer"
,immediatelyShowMarkdownHelp:true
});
}
});
Sign up or log in
StackExchange.ready(function () {
StackExchange.helpers.onClickDraftSave('#login-link');
});
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Post as a guest
Required, but never shown
StackExchange.ready(
function () {
StackExchange.openid.initPostLogin('.new-post-login', 'https%3a%2f%2fmath.stackexchange.com%2fquestions%2f3064255%2fproving-the-statement-l1-is-complete-differently%23new-answer', 'question_page');
}
);
Post as a guest
Required, but never shown
1 Answer
1
active
oldest
votes
1 Answer
1
active
oldest
votes
active
oldest
votes
active
oldest
votes
$begingroup$
Sketch: picking up from our comments, we have $x^{(j)}_nto y_j$ for each $jin mathbb N$.
Step $1$: show that $y=(y_j)in ell^1:$
$|y|:=lim_{Jto infty}sum^J_{j=1}|y_j|=lim_{Jto infty}(sum^J_{j=1}lim_{nto infty}|x^{(j)}_n|)=lim_{Jto infty}lim_{nto infty}(sum^J_{j=1}|x^{(j)}_n|).$
Now, $sum^J_{j=1}|x^{(j)}_n|le sum^{infty}_{j=1}|x^{(j)}_n|=|x_n|le M<infty $ because the original sequence $(x_n)$ is Cauchy, hence bounded. So,
$|y|le lim_{nto infty}M=M.$
Step $2:$ Prove that $|x_n-y|to 0:$
Let $epsilon>0$ and choose an integer $N$ such that $l,n>NRightarrow |x_l-x_n|<epsilon.$
Then, $sum^J_{j=1}|x_n^{(j)}-x_l^{(j)}|le sum^{infty}_{j=1}|x_n^{(j)}-x_l^{(j)}|=|x_n-x_l|<epsilon.$
I'll leave the rest to you: keep $n>N$, fix $J$ and let $lto infty.$ Finally, let $Jto infty.$
$endgroup$
$begingroup$
I have problem in the second part. Please complete it. @Matematleta
$endgroup$
– cmi
Jan 6 at 20:00
$begingroup$
And I have seen this proof earlier. It is kind of escaping proof. It can not be visualized.@Matematleta
$endgroup$
– cmi
Jan 6 at 20:05
$begingroup$
The Cauchy criterion? What? All I am doing is proving that the logical candidate for the limit of the $(x_n)$ is indeed the limit. To do this, you need to show two things 1). it's in $ell^1$. and 2) the sequence $(x_n)$ actually converges to it. I think my sketch is a good roadmap. Suggestion: write the sequence $(x_n)$ in matrix form to see what is going on. The fact that $(x_n)$ is Cauchy in each coordinate is really all you need to know.
$endgroup$
– Matematleta
Jan 6 at 20:06
$begingroup$
without using limit can you show it ?@Matematleta
$endgroup$
– cmi
Jan 6 at 20:43
1
$begingroup$
Your answer helped me to understand what I wanted to understand.@Matematleta
$endgroup$
– cmi
Jan 9 at 4:14
|
show 1 more comment
$begingroup$
Sketch: picking up from our comments, we have $x^{(j)}_nto y_j$ for each $jin mathbb N$.
Step $1$: show that $y=(y_j)in ell^1:$
$|y|:=lim_{Jto infty}sum^J_{j=1}|y_j|=lim_{Jto infty}(sum^J_{j=1}lim_{nto infty}|x^{(j)}_n|)=lim_{Jto infty}lim_{nto infty}(sum^J_{j=1}|x^{(j)}_n|).$
Now, $sum^J_{j=1}|x^{(j)}_n|le sum^{infty}_{j=1}|x^{(j)}_n|=|x_n|le M<infty $ because the original sequence $(x_n)$ is Cauchy, hence bounded. So,
$|y|le lim_{nto infty}M=M.$
Step $2:$ Prove that $|x_n-y|to 0:$
Let $epsilon>0$ and choose an integer $N$ such that $l,n>NRightarrow |x_l-x_n|<epsilon.$
Then, $sum^J_{j=1}|x_n^{(j)}-x_l^{(j)}|le sum^{infty}_{j=1}|x_n^{(j)}-x_l^{(j)}|=|x_n-x_l|<epsilon.$
I'll leave the rest to you: keep $n>N$, fix $J$ and let $lto infty.$ Finally, let $Jto infty.$
$endgroup$
$begingroup$
I have problem in the second part. Please complete it. @Matematleta
$endgroup$
– cmi
Jan 6 at 20:00
$begingroup$
And I have seen this proof earlier. It is kind of escaping proof. It can not be visualized.@Matematleta
$endgroup$
– cmi
Jan 6 at 20:05
$begingroup$
The Cauchy criterion? What? All I am doing is proving that the logical candidate for the limit of the $(x_n)$ is indeed the limit. To do this, you need to show two things 1). it's in $ell^1$. and 2) the sequence $(x_n)$ actually converges to it. I think my sketch is a good roadmap. Suggestion: write the sequence $(x_n)$ in matrix form to see what is going on. The fact that $(x_n)$ is Cauchy in each coordinate is really all you need to know.
$endgroup$
– Matematleta
Jan 6 at 20:06
$begingroup$
without using limit can you show it ?@Matematleta
$endgroup$
– cmi
Jan 6 at 20:43
1
$begingroup$
Your answer helped me to understand what I wanted to understand.@Matematleta
$endgroup$
– cmi
Jan 9 at 4:14
|
show 1 more comment
$begingroup$
Sketch: picking up from our comments, we have $x^{(j)}_nto y_j$ for each $jin mathbb N$.
Step $1$: show that $y=(y_j)in ell^1:$
$|y|:=lim_{Jto infty}sum^J_{j=1}|y_j|=lim_{Jto infty}(sum^J_{j=1}lim_{nto infty}|x^{(j)}_n|)=lim_{Jto infty}lim_{nto infty}(sum^J_{j=1}|x^{(j)}_n|).$
Now, $sum^J_{j=1}|x^{(j)}_n|le sum^{infty}_{j=1}|x^{(j)}_n|=|x_n|le M<infty $ because the original sequence $(x_n)$ is Cauchy, hence bounded. So,
$|y|le lim_{nto infty}M=M.$
Step $2:$ Prove that $|x_n-y|to 0:$
Let $epsilon>0$ and choose an integer $N$ such that $l,n>NRightarrow |x_l-x_n|<epsilon.$
Then, $sum^J_{j=1}|x_n^{(j)}-x_l^{(j)}|le sum^{infty}_{j=1}|x_n^{(j)}-x_l^{(j)}|=|x_n-x_l|<epsilon.$
I'll leave the rest to you: keep $n>N$, fix $J$ and let $lto infty.$ Finally, let $Jto infty.$
$endgroup$
Sketch: picking up from our comments, we have $x^{(j)}_nto y_j$ for each $jin mathbb N$.
Step $1$: show that $y=(y_j)in ell^1:$
$|y|:=lim_{Jto infty}sum^J_{j=1}|y_j|=lim_{Jto infty}(sum^J_{j=1}lim_{nto infty}|x^{(j)}_n|)=lim_{Jto infty}lim_{nto infty}(sum^J_{j=1}|x^{(j)}_n|).$
Now, $sum^J_{j=1}|x^{(j)}_n|le sum^{infty}_{j=1}|x^{(j)}_n|=|x_n|le M<infty $ because the original sequence $(x_n)$ is Cauchy, hence bounded. So,
$|y|le lim_{nto infty}M=M.$
Step $2:$ Prove that $|x_n-y|to 0:$
Let $epsilon>0$ and choose an integer $N$ such that $l,n>NRightarrow |x_l-x_n|<epsilon.$
Then, $sum^J_{j=1}|x_n^{(j)}-x_l^{(j)}|le sum^{infty}_{j=1}|x_n^{(j)}-x_l^{(j)}|=|x_n-x_l|<epsilon.$
I'll leave the rest to you: keep $n>N$, fix $J$ and let $lto infty.$ Finally, let $Jto infty.$
answered Jan 6 at 19:52
MatematletaMatematleta
11.6k2920
11.6k2920
$begingroup$
I have problem in the second part. Please complete it. @Matematleta
$endgroup$
– cmi
Jan 6 at 20:00
$begingroup$
And I have seen this proof earlier. It is kind of escaping proof. It can not be visualized.@Matematleta
$endgroup$
– cmi
Jan 6 at 20:05
$begingroup$
The Cauchy criterion? What? All I am doing is proving that the logical candidate for the limit of the $(x_n)$ is indeed the limit. To do this, you need to show two things 1). it's in $ell^1$. and 2) the sequence $(x_n)$ actually converges to it. I think my sketch is a good roadmap. Suggestion: write the sequence $(x_n)$ in matrix form to see what is going on. The fact that $(x_n)$ is Cauchy in each coordinate is really all you need to know.
$endgroup$
– Matematleta
Jan 6 at 20:06
$begingroup$
without using limit can you show it ?@Matematleta
$endgroup$
– cmi
Jan 6 at 20:43
1
$begingroup$
Your answer helped me to understand what I wanted to understand.@Matematleta
$endgroup$
– cmi
Jan 9 at 4:14
|
show 1 more comment
$begingroup$
I have problem in the second part. Please complete it. @Matematleta
$endgroup$
– cmi
Jan 6 at 20:00
$begingroup$
And I have seen this proof earlier. It is kind of escaping proof. It can not be visualized.@Matematleta
$endgroup$
– cmi
Jan 6 at 20:05
$begingroup$
The Cauchy criterion? What? All I am doing is proving that the logical candidate for the limit of the $(x_n)$ is indeed the limit. To do this, you need to show two things 1). it's in $ell^1$. and 2) the sequence $(x_n)$ actually converges to it. I think my sketch is a good roadmap. Suggestion: write the sequence $(x_n)$ in matrix form to see what is going on. The fact that $(x_n)$ is Cauchy in each coordinate is really all you need to know.
$endgroup$
– Matematleta
Jan 6 at 20:06
$begingroup$
without using limit can you show it ?@Matematleta
$endgroup$
– cmi
Jan 6 at 20:43
1
$begingroup$
Your answer helped me to understand what I wanted to understand.@Matematleta
$endgroup$
– cmi
Jan 9 at 4:14
$begingroup$
I have problem in the second part. Please complete it. @Matematleta
$endgroup$
– cmi
Jan 6 at 20:00
$begingroup$
I have problem in the second part. Please complete it. @Matematleta
$endgroup$
– cmi
Jan 6 at 20:00
$begingroup$
And I have seen this proof earlier. It is kind of escaping proof. It can not be visualized.@Matematleta
$endgroup$
– cmi
Jan 6 at 20:05
$begingroup$
And I have seen this proof earlier. It is kind of escaping proof. It can not be visualized.@Matematleta
$endgroup$
– cmi
Jan 6 at 20:05
$begingroup$
The Cauchy criterion? What? All I am doing is proving that the logical candidate for the limit of the $(x_n)$ is indeed the limit. To do this, you need to show two things 1). it's in $ell^1$. and 2) the sequence $(x_n)$ actually converges to it. I think my sketch is a good roadmap. Suggestion: write the sequence $(x_n)$ in matrix form to see what is going on. The fact that $(x_n)$ is Cauchy in each coordinate is really all you need to know.
$endgroup$
– Matematleta
Jan 6 at 20:06
$begingroup$
The Cauchy criterion? What? All I am doing is proving that the logical candidate for the limit of the $(x_n)$ is indeed the limit. To do this, you need to show two things 1). it's in $ell^1$. and 2) the sequence $(x_n)$ actually converges to it. I think my sketch is a good roadmap. Suggestion: write the sequence $(x_n)$ in matrix form to see what is going on. The fact that $(x_n)$ is Cauchy in each coordinate is really all you need to know.
$endgroup$
– Matematleta
Jan 6 at 20:06
$begingroup$
without using limit can you show it ?@Matematleta
$endgroup$
– cmi
Jan 6 at 20:43
$begingroup$
without using limit can you show it ?@Matematleta
$endgroup$
– cmi
Jan 6 at 20:43
1
1
$begingroup$
Your answer helped me to understand what I wanted to understand.@Matematleta
$endgroup$
– cmi
Jan 9 at 4:14
$begingroup$
Your answer helped me to understand what I wanted to understand.@Matematleta
$endgroup$
– cmi
Jan 9 at 4:14
|
show 1 more comment
Thanks for contributing an answer to Mathematics Stack Exchange!
- Please be sure to answer the question. Provide details and share your research!
But avoid …
- Asking for help, clarification, or responding to other answers.
- Making statements based on opinion; back them up with references or personal experience.
Use MathJax to format equations. MathJax reference.
To learn more, see our tips on writing great answers.
Sign up or log in
StackExchange.ready(function () {
StackExchange.helpers.onClickDraftSave('#login-link');
});
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Post as a guest
Required, but never shown
StackExchange.ready(
function () {
StackExchange.openid.initPostLogin('.new-post-login', 'https%3a%2f%2fmath.stackexchange.com%2fquestions%2f3064255%2fproving-the-statement-l1-is-complete-differently%23new-answer', 'question_page');
}
);
Post as a guest
Required, but never shown
Sign up or log in
StackExchange.ready(function () {
StackExchange.helpers.onClickDraftSave('#login-link');
});
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Post as a guest
Required, but never shown
Sign up or log in
StackExchange.ready(function () {
StackExchange.helpers.onClickDraftSave('#login-link');
});
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Post as a guest
Required, but never shown
Sign up or log in
StackExchange.ready(function () {
StackExchange.helpers.onClickDraftSave('#login-link');
});
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Post as a guest
Required, but never shown
Required, but never shown
Required, but never shown
Required, but never shown
Required, but never shown
Required, but never shown
Required, but never shown
Required, but never shown
Required, but never shown
$begingroup$
if $(x_n)$ is Cauchy, then so are the coordinate sequences $(x^{(j)}_n)$. So if your scalar field is complete, you get a candidate for the limit sequence.
$endgroup$
– Matematleta
Jan 6 at 19:01
$begingroup$
Yea I understood up to the limit point convergence. I have written this in my question. $(y)$ is that sequence. But I have to prove that $(x_n)$ converges to $(y)$.@Matematleta
$endgroup$
– cmi
Jan 6 at 19:04
$begingroup$
Then, I am not sure what you are asking but I have posted a sketch of a proof. If it is not what you are looking for, let me know and I will be happy to delete it.
$endgroup$
– Matematleta
Jan 6 at 19:53
$begingroup$
Please show me how the Cauchy sequence $(x_n)$ converges to $y$.@Matematleta
$endgroup$
– cmi
Jan 6 at 20:01