Limit of product of infinite sequences and infinite series
$begingroup$
If the limit $$lim_{nto infty} n^p u_n =A tag{1}\ pgt 1;A lt infty$$ then I have to show that the limit of the series $sum_{n=0} ^{infty}u_n $ converges. I know that since $n^p$ blows up at infinity $u_n$ must somehow go to zero. I have tried the following:
$$lim_{nto infty} n^p u_n =A tag{2} = lim_{nto infty} n^p cdot lim_{nto infty} u_n \
therefore$$ $$lim_{nto infty}u_n = frac{lim_{nto infty}n^p}{A} tag{3}$$
From there I concluded that $u_n = frac{n^p}{A}$. Then applying the ratio test:
$$lim_{nto infty}frac{u_{n+1}}{u_n} = lim_{nto infty} {}frac{(n+1)^p}{n^p} lt 1 tag{4}$$
but obviously this doesn't work and is wrong. I am not convinced that $(2)$ is correct because I am not sure if the limits ($lim_{n to infty }n^p $ is $infty$ ???) exist. Also $(4)$ doesn't really work I guess because the limit is probably $=1$.
real-analysis sequences-and-series limits
$endgroup$
add a comment |
$begingroup$
If the limit $$lim_{nto infty} n^p u_n =A tag{1}\ pgt 1;A lt infty$$ then I have to show that the limit of the series $sum_{n=0} ^{infty}u_n $ converges. I know that since $n^p$ blows up at infinity $u_n$ must somehow go to zero. I have tried the following:
$$lim_{nto infty} n^p u_n =A tag{2} = lim_{nto infty} n^p cdot lim_{nto infty} u_n \
therefore$$ $$lim_{nto infty}u_n = frac{lim_{nto infty}n^p}{A} tag{3}$$
From there I concluded that $u_n = frac{n^p}{A}$. Then applying the ratio test:
$$lim_{nto infty}frac{u_{n+1}}{u_n} = lim_{nto infty} {}frac{(n+1)^p}{n^p} lt 1 tag{4}$$
but obviously this doesn't work and is wrong. I am not convinced that $(2)$ is correct because I am not sure if the limits ($lim_{n to infty }n^p $ is $infty$ ???) exist. Also $(4)$ doesn't really work I guess because the limit is probably $=1$.
real-analysis sequences-and-series limits
$endgroup$
add a comment |
$begingroup$
If the limit $$lim_{nto infty} n^p u_n =A tag{1}\ pgt 1;A lt infty$$ then I have to show that the limit of the series $sum_{n=0} ^{infty}u_n $ converges. I know that since $n^p$ blows up at infinity $u_n$ must somehow go to zero. I have tried the following:
$$lim_{nto infty} n^p u_n =A tag{2} = lim_{nto infty} n^p cdot lim_{nto infty} u_n \
therefore$$ $$lim_{nto infty}u_n = frac{lim_{nto infty}n^p}{A} tag{3}$$
From there I concluded that $u_n = frac{n^p}{A}$. Then applying the ratio test:
$$lim_{nto infty}frac{u_{n+1}}{u_n} = lim_{nto infty} {}frac{(n+1)^p}{n^p} lt 1 tag{4}$$
but obviously this doesn't work and is wrong. I am not convinced that $(2)$ is correct because I am not sure if the limits ($lim_{n to infty }n^p $ is $infty$ ???) exist. Also $(4)$ doesn't really work I guess because the limit is probably $=1$.
real-analysis sequences-and-series limits
$endgroup$
If the limit $$lim_{nto infty} n^p u_n =A tag{1}\ pgt 1;A lt infty$$ then I have to show that the limit of the series $sum_{n=0} ^{infty}u_n $ converges. I know that since $n^p$ blows up at infinity $u_n$ must somehow go to zero. I have tried the following:
$$lim_{nto infty} n^p u_n =A tag{2} = lim_{nto infty} n^p cdot lim_{nto infty} u_n \
therefore$$ $$lim_{nto infty}u_n = frac{lim_{nto infty}n^p}{A} tag{3}$$
From there I concluded that $u_n = frac{n^p}{A}$. Then applying the ratio test:
$$lim_{nto infty}frac{u_{n+1}}{u_n} = lim_{nto infty} {}frac{(n+1)^p}{n^p} lt 1 tag{4}$$
but obviously this doesn't work and is wrong. I am not convinced that $(2)$ is correct because I am not sure if the limits ($lim_{n to infty }n^p $ is $infty$ ???) exist. Also $(4)$ doesn't really work I guess because the limit is probably $=1$.
real-analysis sequences-and-series limits
real-analysis sequences-and-series limits
edited Jan 25 at 12:20
daljit97
asked Jan 25 at 12:08
daljit97daljit97
178111
178111
add a comment |
add a comment |
3 Answers
3
active
oldest
votes
$begingroup$
First of all, you are doing a lot of illegal stuff there. (2), as you already pointed out, does not work as $n^p$ is not convergent. Then you are essentially dividing by $0$ from (2) to (3) (among another elementary mistake) and after that, you conclude $u_n = frac{n^p}{A}$ from $limlimits_{nto infty}u_n = frac{limlimits_{nto infty}n^p}{A}$, which is obviously not correct, either.
Now for your actual question. Write $$sum_{n=0}^{infty}u_n = sum_{n=0}^{infty} frac{1}{n^p}(n^pu_n).$$
Can you figure it out now?
$endgroup$
$begingroup$
I know that $sum_{n=0} ^ {infty} {frac{1}{n^p}}$ is convergent and I suppose that $sum_{n=0} ^ {infty} {n^p u_n}$ is also convergent?
$endgroup$
– daljit97
Jan 25 at 15:32
$begingroup$
No, $(n^p u_n)$ is just bounded, but that's sufficient.
$endgroup$
– Klaus
Jan 25 at 15:38
$begingroup$
Ok so I can say $sum_{n=0}^{infty}u_n lt sum_{n=0}^{infty} frac{1}{n^p}(M).$ for some $M in mathbb{R}$?
$endgroup$
– daljit97
Jan 25 at 15:43
$begingroup$
Better use an absolute value (i.e. $|u_n|$), but otherwise yes.
$endgroup$
– Klaus
Jan 25 at 15:45
$begingroup$
Ok since $sum_{n=0} ^ {infty} {frac{1}{n^p}}$ converges obviously $M sum_{n=0} ^ {infty} {frac{1}{n^p}}$ converges right?
$endgroup$
– daljit97
Jan 26 at 2:11
add a comment |
$begingroup$
As $lim_{nto infty} n^p u_n =A $ is finite, $(n^p u_n)$ is a bounded sequence (a sequence that converges is bounded). Let's say
$vert n^p u_n vert le B$ with $B >0$.
then you have $$vert u_n vert le frac{B}{n^p}$$ proving that $sum u_n$ is absolutely converging as for $p > 1$, $sum 1/n^p$ converges.
$endgroup$
add a comment |
$begingroup$
$(2)$ and $(3)$ are meaningless. $(4)$ is actually right (except that the limit is equal to $1$), but your derivation isn’t.
So, here, the ratio test is useless. The actual useful assumption is that $n^pu_n$ is convergent. Therefore it is bounded: so $|u_n| leq Mn^{-p}$ for each $n$. Can you take it from there?
$endgroup$
add a 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%2f3087033%2flimit-of-product-of-infinite-sequences-and-infinite-series%23new-answer', 'question_page');
}
);
Post as a guest
Required, but never shown
3 Answers
3
active
oldest
votes
3 Answers
3
active
oldest
votes
active
oldest
votes
active
oldest
votes
$begingroup$
First of all, you are doing a lot of illegal stuff there. (2), as you already pointed out, does not work as $n^p$ is not convergent. Then you are essentially dividing by $0$ from (2) to (3) (among another elementary mistake) and after that, you conclude $u_n = frac{n^p}{A}$ from $limlimits_{nto infty}u_n = frac{limlimits_{nto infty}n^p}{A}$, which is obviously not correct, either.
Now for your actual question. Write $$sum_{n=0}^{infty}u_n = sum_{n=0}^{infty} frac{1}{n^p}(n^pu_n).$$
Can you figure it out now?
$endgroup$
$begingroup$
I know that $sum_{n=0} ^ {infty} {frac{1}{n^p}}$ is convergent and I suppose that $sum_{n=0} ^ {infty} {n^p u_n}$ is also convergent?
$endgroup$
– daljit97
Jan 25 at 15:32
$begingroup$
No, $(n^p u_n)$ is just bounded, but that's sufficient.
$endgroup$
– Klaus
Jan 25 at 15:38
$begingroup$
Ok so I can say $sum_{n=0}^{infty}u_n lt sum_{n=0}^{infty} frac{1}{n^p}(M).$ for some $M in mathbb{R}$?
$endgroup$
– daljit97
Jan 25 at 15:43
$begingroup$
Better use an absolute value (i.e. $|u_n|$), but otherwise yes.
$endgroup$
– Klaus
Jan 25 at 15:45
$begingroup$
Ok since $sum_{n=0} ^ {infty} {frac{1}{n^p}}$ converges obviously $M sum_{n=0} ^ {infty} {frac{1}{n^p}}$ converges right?
$endgroup$
– daljit97
Jan 26 at 2:11
add a comment |
$begingroup$
First of all, you are doing a lot of illegal stuff there. (2), as you already pointed out, does not work as $n^p$ is not convergent. Then you are essentially dividing by $0$ from (2) to (3) (among another elementary mistake) and after that, you conclude $u_n = frac{n^p}{A}$ from $limlimits_{nto infty}u_n = frac{limlimits_{nto infty}n^p}{A}$, which is obviously not correct, either.
Now for your actual question. Write $$sum_{n=0}^{infty}u_n = sum_{n=0}^{infty} frac{1}{n^p}(n^pu_n).$$
Can you figure it out now?
$endgroup$
$begingroup$
I know that $sum_{n=0} ^ {infty} {frac{1}{n^p}}$ is convergent and I suppose that $sum_{n=0} ^ {infty} {n^p u_n}$ is also convergent?
$endgroup$
– daljit97
Jan 25 at 15:32
$begingroup$
No, $(n^p u_n)$ is just bounded, but that's sufficient.
$endgroup$
– Klaus
Jan 25 at 15:38
$begingroup$
Ok so I can say $sum_{n=0}^{infty}u_n lt sum_{n=0}^{infty} frac{1}{n^p}(M).$ for some $M in mathbb{R}$?
$endgroup$
– daljit97
Jan 25 at 15:43
$begingroup$
Better use an absolute value (i.e. $|u_n|$), but otherwise yes.
$endgroup$
– Klaus
Jan 25 at 15:45
$begingroup$
Ok since $sum_{n=0} ^ {infty} {frac{1}{n^p}}$ converges obviously $M sum_{n=0} ^ {infty} {frac{1}{n^p}}$ converges right?
$endgroup$
– daljit97
Jan 26 at 2:11
add a comment |
$begingroup$
First of all, you are doing a lot of illegal stuff there. (2), as you already pointed out, does not work as $n^p$ is not convergent. Then you are essentially dividing by $0$ from (2) to (3) (among another elementary mistake) and after that, you conclude $u_n = frac{n^p}{A}$ from $limlimits_{nto infty}u_n = frac{limlimits_{nto infty}n^p}{A}$, which is obviously not correct, either.
Now for your actual question. Write $$sum_{n=0}^{infty}u_n = sum_{n=0}^{infty} frac{1}{n^p}(n^pu_n).$$
Can you figure it out now?
$endgroup$
First of all, you are doing a lot of illegal stuff there. (2), as you already pointed out, does not work as $n^p$ is not convergent. Then you are essentially dividing by $0$ from (2) to (3) (among another elementary mistake) and after that, you conclude $u_n = frac{n^p}{A}$ from $limlimits_{nto infty}u_n = frac{limlimits_{nto infty}n^p}{A}$, which is obviously not correct, either.
Now for your actual question. Write $$sum_{n=0}^{infty}u_n = sum_{n=0}^{infty} frac{1}{n^p}(n^pu_n).$$
Can you figure it out now?
edited Jan 25 at 12:23
answered Jan 25 at 12:17
KlausKlaus
2,56713
2,56713
$begingroup$
I know that $sum_{n=0} ^ {infty} {frac{1}{n^p}}$ is convergent and I suppose that $sum_{n=0} ^ {infty} {n^p u_n}$ is also convergent?
$endgroup$
– daljit97
Jan 25 at 15:32
$begingroup$
No, $(n^p u_n)$ is just bounded, but that's sufficient.
$endgroup$
– Klaus
Jan 25 at 15:38
$begingroup$
Ok so I can say $sum_{n=0}^{infty}u_n lt sum_{n=0}^{infty} frac{1}{n^p}(M).$ for some $M in mathbb{R}$?
$endgroup$
– daljit97
Jan 25 at 15:43
$begingroup$
Better use an absolute value (i.e. $|u_n|$), but otherwise yes.
$endgroup$
– Klaus
Jan 25 at 15:45
$begingroup$
Ok since $sum_{n=0} ^ {infty} {frac{1}{n^p}}$ converges obviously $M sum_{n=0} ^ {infty} {frac{1}{n^p}}$ converges right?
$endgroup$
– daljit97
Jan 26 at 2:11
add a comment |
$begingroup$
I know that $sum_{n=0} ^ {infty} {frac{1}{n^p}}$ is convergent and I suppose that $sum_{n=0} ^ {infty} {n^p u_n}$ is also convergent?
$endgroup$
– daljit97
Jan 25 at 15:32
$begingroup$
No, $(n^p u_n)$ is just bounded, but that's sufficient.
$endgroup$
– Klaus
Jan 25 at 15:38
$begingroup$
Ok so I can say $sum_{n=0}^{infty}u_n lt sum_{n=0}^{infty} frac{1}{n^p}(M).$ for some $M in mathbb{R}$?
$endgroup$
– daljit97
Jan 25 at 15:43
$begingroup$
Better use an absolute value (i.e. $|u_n|$), but otherwise yes.
$endgroup$
– Klaus
Jan 25 at 15:45
$begingroup$
Ok since $sum_{n=0} ^ {infty} {frac{1}{n^p}}$ converges obviously $M sum_{n=0} ^ {infty} {frac{1}{n^p}}$ converges right?
$endgroup$
– daljit97
Jan 26 at 2:11
$begingroup$
I know that $sum_{n=0} ^ {infty} {frac{1}{n^p}}$ is convergent and I suppose that $sum_{n=0} ^ {infty} {n^p u_n}$ is also convergent?
$endgroup$
– daljit97
Jan 25 at 15:32
$begingroup$
I know that $sum_{n=0} ^ {infty} {frac{1}{n^p}}$ is convergent and I suppose that $sum_{n=0} ^ {infty} {n^p u_n}$ is also convergent?
$endgroup$
– daljit97
Jan 25 at 15:32
$begingroup$
No, $(n^p u_n)$ is just bounded, but that's sufficient.
$endgroup$
– Klaus
Jan 25 at 15:38
$begingroup$
No, $(n^p u_n)$ is just bounded, but that's sufficient.
$endgroup$
– Klaus
Jan 25 at 15:38
$begingroup$
Ok so I can say $sum_{n=0}^{infty}u_n lt sum_{n=0}^{infty} frac{1}{n^p}(M).$ for some $M in mathbb{R}$?
$endgroup$
– daljit97
Jan 25 at 15:43
$begingroup$
Ok so I can say $sum_{n=0}^{infty}u_n lt sum_{n=0}^{infty} frac{1}{n^p}(M).$ for some $M in mathbb{R}$?
$endgroup$
– daljit97
Jan 25 at 15:43
$begingroup$
Better use an absolute value (i.e. $|u_n|$), but otherwise yes.
$endgroup$
– Klaus
Jan 25 at 15:45
$begingroup$
Better use an absolute value (i.e. $|u_n|$), but otherwise yes.
$endgroup$
– Klaus
Jan 25 at 15:45
$begingroup$
Ok since $sum_{n=0} ^ {infty} {frac{1}{n^p}}$ converges obviously $M sum_{n=0} ^ {infty} {frac{1}{n^p}}$ converges right?
$endgroup$
– daljit97
Jan 26 at 2:11
$begingroup$
Ok since $sum_{n=0} ^ {infty} {frac{1}{n^p}}$ converges obviously $M sum_{n=0} ^ {infty} {frac{1}{n^p}}$ converges right?
$endgroup$
– daljit97
Jan 26 at 2:11
add a comment |
$begingroup$
As $lim_{nto infty} n^p u_n =A $ is finite, $(n^p u_n)$ is a bounded sequence (a sequence that converges is bounded). Let's say
$vert n^p u_n vert le B$ with $B >0$.
then you have $$vert u_n vert le frac{B}{n^p}$$ proving that $sum u_n$ is absolutely converging as for $p > 1$, $sum 1/n^p$ converges.
$endgroup$
add a comment |
$begingroup$
As $lim_{nto infty} n^p u_n =A $ is finite, $(n^p u_n)$ is a bounded sequence (a sequence that converges is bounded). Let's say
$vert n^p u_n vert le B$ with $B >0$.
then you have $$vert u_n vert le frac{B}{n^p}$$ proving that $sum u_n$ is absolutely converging as for $p > 1$, $sum 1/n^p$ converges.
$endgroup$
add a comment |
$begingroup$
As $lim_{nto infty} n^p u_n =A $ is finite, $(n^p u_n)$ is a bounded sequence (a sequence that converges is bounded). Let's say
$vert n^p u_n vert le B$ with $B >0$.
then you have $$vert u_n vert le frac{B}{n^p}$$ proving that $sum u_n$ is absolutely converging as for $p > 1$, $sum 1/n^p$ converges.
$endgroup$
As $lim_{nto infty} n^p u_n =A $ is finite, $(n^p u_n)$ is a bounded sequence (a sequence that converges is bounded). Let's say
$vert n^p u_n vert le B$ with $B >0$.
then you have $$vert u_n vert le frac{B}{n^p}$$ proving that $sum u_n$ is absolutely converging as for $p > 1$, $sum 1/n^p$ converges.
answered Jan 25 at 12:17


mathcounterexamples.netmathcounterexamples.net
27k22158
27k22158
add a comment |
add a comment |
$begingroup$
$(2)$ and $(3)$ are meaningless. $(4)$ is actually right (except that the limit is equal to $1$), but your derivation isn’t.
So, here, the ratio test is useless. The actual useful assumption is that $n^pu_n$ is convergent. Therefore it is bounded: so $|u_n| leq Mn^{-p}$ for each $n$. Can you take it from there?
$endgroup$
add a comment |
$begingroup$
$(2)$ and $(3)$ are meaningless. $(4)$ is actually right (except that the limit is equal to $1$), but your derivation isn’t.
So, here, the ratio test is useless. The actual useful assumption is that $n^pu_n$ is convergent. Therefore it is bounded: so $|u_n| leq Mn^{-p}$ for each $n$. Can you take it from there?
$endgroup$
add a comment |
$begingroup$
$(2)$ and $(3)$ are meaningless. $(4)$ is actually right (except that the limit is equal to $1$), but your derivation isn’t.
So, here, the ratio test is useless. The actual useful assumption is that $n^pu_n$ is convergent. Therefore it is bounded: so $|u_n| leq Mn^{-p}$ for each $n$. Can you take it from there?
$endgroup$
$(2)$ and $(3)$ are meaningless. $(4)$ is actually right (except that the limit is equal to $1$), but your derivation isn’t.
So, here, the ratio test is useless. The actual useful assumption is that $n^pu_n$ is convergent. Therefore it is bounded: so $|u_n| leq Mn^{-p}$ for each $n$. Can you take it from there?
answered Jan 25 at 12:21
MindlackMindlack
4,920211
4,920211
add a comment |
add a 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%2f3087033%2flimit-of-product-of-infinite-sequences-and-infinite-series%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