If $sum a_{n}$ is a series of positive terms and $b_{n} = (a_{1}+a_{2}+a_{3}+ dots +a_{n})/n$ then $sum...
$begingroup$
If $sum a_{n}$ is a series of positive terms and $b_{n} = (a_{1}+a_{2}+a_{3}+ dots +a_{n})/n$ then $sum b_{n}$ is divergent. (True/false)
The statement is given true with a proof in my book.
But I'm confused.
If I take the series $sum frac{1}{n^{2}}, $ for which sequence of partial sum is $a_{n} = (1)+(1/4)+(1/9)+(1/16)+ dots$ then,sequence of partial sum for $sum {b_{n}} $ will be $b_{n} = (1/2)+(1/8)+(1/18)+ dots$ which is convergent.
Then why is this statement true?
sequences-and-series
$endgroup$
add a comment |
$begingroup$
If $sum a_{n}$ is a series of positive terms and $b_{n} = (a_{1}+a_{2}+a_{3}+ dots +a_{n})/n$ then $sum b_{n}$ is divergent. (True/false)
The statement is given true with a proof in my book.
But I'm confused.
If I take the series $sum frac{1}{n^{2}}, $ for which sequence of partial sum is $a_{n} = (1)+(1/4)+(1/9)+(1/16)+ dots$ then,sequence of partial sum for $sum {b_{n}} $ will be $b_{n} = (1/2)+(1/8)+(1/18)+ dots$ which is convergent.
Then why is this statement true?
sequences-and-series
$endgroup$
add a comment |
$begingroup$
If $sum a_{n}$ is a series of positive terms and $b_{n} = (a_{1}+a_{2}+a_{3}+ dots +a_{n})/n$ then $sum b_{n}$ is divergent. (True/false)
The statement is given true with a proof in my book.
But I'm confused.
If I take the series $sum frac{1}{n^{2}}, $ for which sequence of partial sum is $a_{n} = (1)+(1/4)+(1/9)+(1/16)+ dots$ then,sequence of partial sum for $sum {b_{n}} $ will be $b_{n} = (1/2)+(1/8)+(1/18)+ dots$ which is convergent.
Then why is this statement true?
sequences-and-series
$endgroup$
If $sum a_{n}$ is a series of positive terms and $b_{n} = (a_{1}+a_{2}+a_{3}+ dots +a_{n})/n$ then $sum b_{n}$ is divergent. (True/false)
The statement is given true with a proof in my book.
But I'm confused.
If I take the series $sum frac{1}{n^{2}}, $ for which sequence of partial sum is $a_{n} = (1)+(1/4)+(1/9)+(1/16)+ dots$ then,sequence of partial sum for $sum {b_{n}} $ will be $b_{n} = (1/2)+(1/8)+(1/18)+ dots$ which is convergent.
Then why is this statement true?
sequences-and-series
sequences-and-series
asked Jan 7 at 21:03
MathsaddictMathsaddict
3619
3619
add a comment |
add a comment |
2 Answers
2
active
oldest
votes
$begingroup$
This follows by series comparison. Since $b_n ge frac{a_1}{n}$, the terms of the series $sum_n b_n$ are bounded from below by a multiple of the harmonic series, which is divergent,
$endgroup$
$begingroup$
I was just adding my answer when you posted yours. As they're basically the same, although I provide a bit more detail, I can delete mine if you wish.
$endgroup$
– John Omielan
Jan 7 at 21:15
add a comment |
$begingroup$
Given that $a_i$ are all positive and
$$b_n = left(a_1 + a_2 + a_3 + cdots + a_nright)/n tag{1}label{eq1}$$
Then note that $b_n ge frac{a_1}{n}$ for all $n ge 1$. Thus,
$$sum_{i = 1}^{n} b_i ge sum_{i = 1}^{n} frac{a_1}{n} = a_1 sum_{i = 1}^{n} frac{1}{n} tag{2}label{eq2}$$
Note that the latter part is the harmonic series, which diverges.
$endgroup$
$begingroup$
Yes, I have read this proof. But, what is wrong with my example?
$endgroup$
– Mathsaddict
Jan 7 at 21:23
$begingroup$
Note that $sum frac{1}{n}$ diverges, but $sum frac{1}{n^2}$ converges, so you can select $a_i$ for the latter case which will allow $sum b_i$ to converge.
$endgroup$
– John Omielan
Jan 7 at 21:32
1
$begingroup$
@Mathsaddict I misread your original text. Using $a_i = frac{1}{i^2}$, then $b_1 = frac{a_1}{1} = 1$, not $frac{1}{2}$. Next, $b_2 = frac{a_1 + a_2}{2} = frac{1 + 1/4}{2} = frac{5}{8}$, not $frac{1}{8}$. If you continue, you will see the next values are larger than you indicated, and don't become small enough, fast enough, for the sum to converge, as another answer here already states.
$endgroup$
– John Omielan
Jan 7 at 21:41
1
$begingroup$
yes, I calculated $b_{n}$ incorrectly. So, this series $sum b_{n}$ will always be divergent. It doesn't matter if $sum a_{n}$ is convergent or not. Right?
$endgroup$
– Mathsaddict
Jan 8 at 1:48
$begingroup$
@Mathsaddict Yes, you are correct. That is what my proof, and basically the same one of Hans Engler, shows.
$endgroup$
– John Omielan
Jan 8 at 5:46
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%2f3065485%2fif-sum-a-n-is-a-series-of-positive-terms-and-b-n-a-1a-2a-3%23new-answer', 'question_page');
}
);
Post as a guest
Required, but never shown
2 Answers
2
active
oldest
votes
2 Answers
2
active
oldest
votes
active
oldest
votes
active
oldest
votes
$begingroup$
This follows by series comparison. Since $b_n ge frac{a_1}{n}$, the terms of the series $sum_n b_n$ are bounded from below by a multiple of the harmonic series, which is divergent,
$endgroup$
$begingroup$
I was just adding my answer when you posted yours. As they're basically the same, although I provide a bit more detail, I can delete mine if you wish.
$endgroup$
– John Omielan
Jan 7 at 21:15
add a comment |
$begingroup$
This follows by series comparison. Since $b_n ge frac{a_1}{n}$, the terms of the series $sum_n b_n$ are bounded from below by a multiple of the harmonic series, which is divergent,
$endgroup$
$begingroup$
I was just adding my answer when you posted yours. As they're basically the same, although I provide a bit more detail, I can delete mine if you wish.
$endgroup$
– John Omielan
Jan 7 at 21:15
add a comment |
$begingroup$
This follows by series comparison. Since $b_n ge frac{a_1}{n}$, the terms of the series $sum_n b_n$ are bounded from below by a multiple of the harmonic series, which is divergent,
$endgroup$
This follows by series comparison. Since $b_n ge frac{a_1}{n}$, the terms of the series $sum_n b_n$ are bounded from below by a multiple of the harmonic series, which is divergent,
answered Jan 7 at 21:12
Hans EnglerHans Engler
10.5k11836
10.5k11836
$begingroup$
I was just adding my answer when you posted yours. As they're basically the same, although I provide a bit more detail, I can delete mine if you wish.
$endgroup$
– John Omielan
Jan 7 at 21:15
add a comment |
$begingroup$
I was just adding my answer when you posted yours. As they're basically the same, although I provide a bit more detail, I can delete mine if you wish.
$endgroup$
– John Omielan
Jan 7 at 21:15
$begingroup$
I was just adding my answer when you posted yours. As they're basically the same, although I provide a bit more detail, I can delete mine if you wish.
$endgroup$
– John Omielan
Jan 7 at 21:15
$begingroup$
I was just adding my answer when you posted yours. As they're basically the same, although I provide a bit more detail, I can delete mine if you wish.
$endgroup$
– John Omielan
Jan 7 at 21:15
add a comment |
$begingroup$
Given that $a_i$ are all positive and
$$b_n = left(a_1 + a_2 + a_3 + cdots + a_nright)/n tag{1}label{eq1}$$
Then note that $b_n ge frac{a_1}{n}$ for all $n ge 1$. Thus,
$$sum_{i = 1}^{n} b_i ge sum_{i = 1}^{n} frac{a_1}{n} = a_1 sum_{i = 1}^{n} frac{1}{n} tag{2}label{eq2}$$
Note that the latter part is the harmonic series, which diverges.
$endgroup$
$begingroup$
Yes, I have read this proof. But, what is wrong with my example?
$endgroup$
– Mathsaddict
Jan 7 at 21:23
$begingroup$
Note that $sum frac{1}{n}$ diverges, but $sum frac{1}{n^2}$ converges, so you can select $a_i$ for the latter case which will allow $sum b_i$ to converge.
$endgroup$
– John Omielan
Jan 7 at 21:32
1
$begingroup$
@Mathsaddict I misread your original text. Using $a_i = frac{1}{i^2}$, then $b_1 = frac{a_1}{1} = 1$, not $frac{1}{2}$. Next, $b_2 = frac{a_1 + a_2}{2} = frac{1 + 1/4}{2} = frac{5}{8}$, not $frac{1}{8}$. If you continue, you will see the next values are larger than you indicated, and don't become small enough, fast enough, for the sum to converge, as another answer here already states.
$endgroup$
– John Omielan
Jan 7 at 21:41
1
$begingroup$
yes, I calculated $b_{n}$ incorrectly. So, this series $sum b_{n}$ will always be divergent. It doesn't matter if $sum a_{n}$ is convergent or not. Right?
$endgroup$
– Mathsaddict
Jan 8 at 1:48
$begingroup$
@Mathsaddict Yes, you are correct. That is what my proof, and basically the same one of Hans Engler, shows.
$endgroup$
– John Omielan
Jan 8 at 5:46
add a comment |
$begingroup$
Given that $a_i$ are all positive and
$$b_n = left(a_1 + a_2 + a_3 + cdots + a_nright)/n tag{1}label{eq1}$$
Then note that $b_n ge frac{a_1}{n}$ for all $n ge 1$. Thus,
$$sum_{i = 1}^{n} b_i ge sum_{i = 1}^{n} frac{a_1}{n} = a_1 sum_{i = 1}^{n} frac{1}{n} tag{2}label{eq2}$$
Note that the latter part is the harmonic series, which diverges.
$endgroup$
$begingroup$
Yes, I have read this proof. But, what is wrong with my example?
$endgroup$
– Mathsaddict
Jan 7 at 21:23
$begingroup$
Note that $sum frac{1}{n}$ diverges, but $sum frac{1}{n^2}$ converges, so you can select $a_i$ for the latter case which will allow $sum b_i$ to converge.
$endgroup$
– John Omielan
Jan 7 at 21:32
1
$begingroup$
@Mathsaddict I misread your original text. Using $a_i = frac{1}{i^2}$, then $b_1 = frac{a_1}{1} = 1$, not $frac{1}{2}$. Next, $b_2 = frac{a_1 + a_2}{2} = frac{1 + 1/4}{2} = frac{5}{8}$, not $frac{1}{8}$. If you continue, you will see the next values are larger than you indicated, and don't become small enough, fast enough, for the sum to converge, as another answer here already states.
$endgroup$
– John Omielan
Jan 7 at 21:41
1
$begingroup$
yes, I calculated $b_{n}$ incorrectly. So, this series $sum b_{n}$ will always be divergent. It doesn't matter if $sum a_{n}$ is convergent or not. Right?
$endgroup$
– Mathsaddict
Jan 8 at 1:48
$begingroup$
@Mathsaddict Yes, you are correct. That is what my proof, and basically the same one of Hans Engler, shows.
$endgroup$
– John Omielan
Jan 8 at 5:46
add a comment |
$begingroup$
Given that $a_i$ are all positive and
$$b_n = left(a_1 + a_2 + a_3 + cdots + a_nright)/n tag{1}label{eq1}$$
Then note that $b_n ge frac{a_1}{n}$ for all $n ge 1$. Thus,
$$sum_{i = 1}^{n} b_i ge sum_{i = 1}^{n} frac{a_1}{n} = a_1 sum_{i = 1}^{n} frac{1}{n} tag{2}label{eq2}$$
Note that the latter part is the harmonic series, which diverges.
$endgroup$
Given that $a_i$ are all positive and
$$b_n = left(a_1 + a_2 + a_3 + cdots + a_nright)/n tag{1}label{eq1}$$
Then note that $b_n ge frac{a_1}{n}$ for all $n ge 1$. Thus,
$$sum_{i = 1}^{n} b_i ge sum_{i = 1}^{n} frac{a_1}{n} = a_1 sum_{i = 1}^{n} frac{1}{n} tag{2}label{eq2}$$
Note that the latter part is the harmonic series, which diverges.
answered Jan 7 at 21:13
John OmielanJohn Omielan
2,779212
2,779212
$begingroup$
Yes, I have read this proof. But, what is wrong with my example?
$endgroup$
– Mathsaddict
Jan 7 at 21:23
$begingroup$
Note that $sum frac{1}{n}$ diverges, but $sum frac{1}{n^2}$ converges, so you can select $a_i$ for the latter case which will allow $sum b_i$ to converge.
$endgroup$
– John Omielan
Jan 7 at 21:32
1
$begingroup$
@Mathsaddict I misread your original text. Using $a_i = frac{1}{i^2}$, then $b_1 = frac{a_1}{1} = 1$, not $frac{1}{2}$. Next, $b_2 = frac{a_1 + a_2}{2} = frac{1 + 1/4}{2} = frac{5}{8}$, not $frac{1}{8}$. If you continue, you will see the next values are larger than you indicated, and don't become small enough, fast enough, for the sum to converge, as another answer here already states.
$endgroup$
– John Omielan
Jan 7 at 21:41
1
$begingroup$
yes, I calculated $b_{n}$ incorrectly. So, this series $sum b_{n}$ will always be divergent. It doesn't matter if $sum a_{n}$ is convergent or not. Right?
$endgroup$
– Mathsaddict
Jan 8 at 1:48
$begingroup$
@Mathsaddict Yes, you are correct. That is what my proof, and basically the same one of Hans Engler, shows.
$endgroup$
– John Omielan
Jan 8 at 5:46
add a comment |
$begingroup$
Yes, I have read this proof. But, what is wrong with my example?
$endgroup$
– Mathsaddict
Jan 7 at 21:23
$begingroup$
Note that $sum frac{1}{n}$ diverges, but $sum frac{1}{n^2}$ converges, so you can select $a_i$ for the latter case which will allow $sum b_i$ to converge.
$endgroup$
– John Omielan
Jan 7 at 21:32
1
$begingroup$
@Mathsaddict I misread your original text. Using $a_i = frac{1}{i^2}$, then $b_1 = frac{a_1}{1} = 1$, not $frac{1}{2}$. Next, $b_2 = frac{a_1 + a_2}{2} = frac{1 + 1/4}{2} = frac{5}{8}$, not $frac{1}{8}$. If you continue, you will see the next values are larger than you indicated, and don't become small enough, fast enough, for the sum to converge, as another answer here already states.
$endgroup$
– John Omielan
Jan 7 at 21:41
1
$begingroup$
yes, I calculated $b_{n}$ incorrectly. So, this series $sum b_{n}$ will always be divergent. It doesn't matter if $sum a_{n}$ is convergent or not. Right?
$endgroup$
– Mathsaddict
Jan 8 at 1:48
$begingroup$
@Mathsaddict Yes, you are correct. That is what my proof, and basically the same one of Hans Engler, shows.
$endgroup$
– John Omielan
Jan 8 at 5:46
$begingroup$
Yes, I have read this proof. But, what is wrong with my example?
$endgroup$
– Mathsaddict
Jan 7 at 21:23
$begingroup$
Yes, I have read this proof. But, what is wrong with my example?
$endgroup$
– Mathsaddict
Jan 7 at 21:23
$begingroup$
Note that $sum frac{1}{n}$ diverges, but $sum frac{1}{n^2}$ converges, so you can select $a_i$ for the latter case which will allow $sum b_i$ to converge.
$endgroup$
– John Omielan
Jan 7 at 21:32
$begingroup$
Note that $sum frac{1}{n}$ diverges, but $sum frac{1}{n^2}$ converges, so you can select $a_i$ for the latter case which will allow $sum b_i$ to converge.
$endgroup$
– John Omielan
Jan 7 at 21:32
1
1
$begingroup$
@Mathsaddict I misread your original text. Using $a_i = frac{1}{i^2}$, then $b_1 = frac{a_1}{1} = 1$, not $frac{1}{2}$. Next, $b_2 = frac{a_1 + a_2}{2} = frac{1 + 1/4}{2} = frac{5}{8}$, not $frac{1}{8}$. If you continue, you will see the next values are larger than you indicated, and don't become small enough, fast enough, for the sum to converge, as another answer here already states.
$endgroup$
– John Omielan
Jan 7 at 21:41
$begingroup$
@Mathsaddict I misread your original text. Using $a_i = frac{1}{i^2}$, then $b_1 = frac{a_1}{1} = 1$, not $frac{1}{2}$. Next, $b_2 = frac{a_1 + a_2}{2} = frac{1 + 1/4}{2} = frac{5}{8}$, not $frac{1}{8}$. If you continue, you will see the next values are larger than you indicated, and don't become small enough, fast enough, for the sum to converge, as another answer here already states.
$endgroup$
– John Omielan
Jan 7 at 21:41
1
1
$begingroup$
yes, I calculated $b_{n}$ incorrectly. So, this series $sum b_{n}$ will always be divergent. It doesn't matter if $sum a_{n}$ is convergent or not. Right?
$endgroup$
– Mathsaddict
Jan 8 at 1:48
$begingroup$
yes, I calculated $b_{n}$ incorrectly. So, this series $sum b_{n}$ will always be divergent. It doesn't matter if $sum a_{n}$ is convergent or not. Right?
$endgroup$
– Mathsaddict
Jan 8 at 1:48
$begingroup$
@Mathsaddict Yes, you are correct. That is what my proof, and basically the same one of Hans Engler, shows.
$endgroup$
– John Omielan
Jan 8 at 5:46
$begingroup$
@Mathsaddict Yes, you are correct. That is what my proof, and basically the same one of Hans Engler, shows.
$endgroup$
– John Omielan
Jan 8 at 5:46
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%2f3065485%2fif-sum-a-n-is-a-series-of-positive-terms-and-b-n-a-1a-2a-3%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