Divergence of series $sum frac{k}{2k+1}$
Consider the infinite series denoted by:
$$sum frac{k}{2k+1}$$
starting at some arbitrary index. We wish to prove that this diverges. We will see what the limit of the general term $a_n$ is, if this does not go to zero, the series diverges by the limit test.
$$lim_{k rightarrow infty} frac{k}{2k+1}=lim_{k rightarrow infty} frac{1}{2+frac{1}{k}}= frac{1}{2+0}=frac{1}{2}$$
By using the fraction limit theorem $(lim frac{A}{B}= frac{lim A}{lim B})$ and sum limit theorem respectively. We conclude that the series diverges as $a_n
not rightarrow 0$.
Is this correct?
I wanted to also try the following proof for $kgeq 1$:
$$ frac{k}{2k+1} > frac{1}{2k+1} geq frac{1}{2k+k} = frac{1}{3} frac{1}{k} $$
Now we simply get the harmonic series, but every factor is multiplied by $frac{1}{3}$, is there still some result we can use about multiplying divergent series by nonzero nonnegative constants?
calculus real-analysis sequences-and-series proof-verification
|
show 2 more comments
Consider the infinite series denoted by:
$$sum frac{k}{2k+1}$$
starting at some arbitrary index. We wish to prove that this diverges. We will see what the limit of the general term $a_n$ is, if this does not go to zero, the series diverges by the limit test.
$$lim_{k rightarrow infty} frac{k}{2k+1}=lim_{k rightarrow infty} frac{1}{2+frac{1}{k}}= frac{1}{2+0}=frac{1}{2}$$
By using the fraction limit theorem $(lim frac{A}{B}= frac{lim A}{lim B})$ and sum limit theorem respectively. We conclude that the series diverges as $a_n
not rightarrow 0$.
Is this correct?
I wanted to also try the following proof for $kgeq 1$:
$$ frac{k}{2k+1} > frac{1}{2k+1} geq frac{1}{2k+k} = frac{1}{3} frac{1}{k} $$
Now we simply get the harmonic series, but every factor is multiplied by $frac{1}{3}$, is there still some result we can use about multiplying divergent series by nonzero nonnegative constants?
calculus real-analysis sequences-and-series proof-verification
1
That first proof looks good, and is how I would solve this problem.
– Dave
Nov 27 '18 at 22:22
Both ways are fine, but what do you mean with "the fraction limit theorem"?
– gimusi
Nov 27 '18 at 22:25
One of the standard results for limits is that if we take a limit of a fraction of two sequences, it is the same as the fraction of limits.
– Wesley Strik
Nov 27 '18 at 22:27
1
@WesleyGroupshaveFeelingsToo Ah ok you are referring to that one $$ lim_{k rightarrow infty} frac{1}{2+frac{1}{k}}$$ That's fine!
– gimusi
Nov 27 '18 at 22:28
In my book a friendly introduction to analysis by Kosmala, it is referred to 2.2.1C
– Wesley Strik
Nov 27 '18 at 22:28
|
show 2 more comments
Consider the infinite series denoted by:
$$sum frac{k}{2k+1}$$
starting at some arbitrary index. We wish to prove that this diverges. We will see what the limit of the general term $a_n$ is, if this does not go to zero, the series diverges by the limit test.
$$lim_{k rightarrow infty} frac{k}{2k+1}=lim_{k rightarrow infty} frac{1}{2+frac{1}{k}}= frac{1}{2+0}=frac{1}{2}$$
By using the fraction limit theorem $(lim frac{A}{B}= frac{lim A}{lim B})$ and sum limit theorem respectively. We conclude that the series diverges as $a_n
not rightarrow 0$.
Is this correct?
I wanted to also try the following proof for $kgeq 1$:
$$ frac{k}{2k+1} > frac{1}{2k+1} geq frac{1}{2k+k} = frac{1}{3} frac{1}{k} $$
Now we simply get the harmonic series, but every factor is multiplied by $frac{1}{3}$, is there still some result we can use about multiplying divergent series by nonzero nonnegative constants?
calculus real-analysis sequences-and-series proof-verification
Consider the infinite series denoted by:
$$sum frac{k}{2k+1}$$
starting at some arbitrary index. We wish to prove that this diverges. We will see what the limit of the general term $a_n$ is, if this does not go to zero, the series diverges by the limit test.
$$lim_{k rightarrow infty} frac{k}{2k+1}=lim_{k rightarrow infty} frac{1}{2+frac{1}{k}}= frac{1}{2+0}=frac{1}{2}$$
By using the fraction limit theorem $(lim frac{A}{B}= frac{lim A}{lim B})$ and sum limit theorem respectively. We conclude that the series diverges as $a_n
not rightarrow 0$.
Is this correct?
I wanted to also try the following proof for $kgeq 1$:
$$ frac{k}{2k+1} > frac{1}{2k+1} geq frac{1}{2k+k} = frac{1}{3} frac{1}{k} $$
Now we simply get the harmonic series, but every factor is multiplied by $frac{1}{3}$, is there still some result we can use about multiplying divergent series by nonzero nonnegative constants?
calculus real-analysis sequences-and-series proof-verification
calculus real-analysis sequences-and-series proof-verification
edited Nov 27 '18 at 22:23
Wesley Strik
asked Nov 27 '18 at 22:20
Wesley StrikWesley Strik
1,623423
1,623423
1
That first proof looks good, and is how I would solve this problem.
– Dave
Nov 27 '18 at 22:22
Both ways are fine, but what do you mean with "the fraction limit theorem"?
– gimusi
Nov 27 '18 at 22:25
One of the standard results for limits is that if we take a limit of a fraction of two sequences, it is the same as the fraction of limits.
– Wesley Strik
Nov 27 '18 at 22:27
1
@WesleyGroupshaveFeelingsToo Ah ok you are referring to that one $$ lim_{k rightarrow infty} frac{1}{2+frac{1}{k}}$$ That's fine!
– gimusi
Nov 27 '18 at 22:28
In my book a friendly introduction to analysis by Kosmala, it is referred to 2.2.1C
– Wesley Strik
Nov 27 '18 at 22:28
|
show 2 more comments
1
That first proof looks good, and is how I would solve this problem.
– Dave
Nov 27 '18 at 22:22
Both ways are fine, but what do you mean with "the fraction limit theorem"?
– gimusi
Nov 27 '18 at 22:25
One of the standard results for limits is that if we take a limit of a fraction of two sequences, it is the same as the fraction of limits.
– Wesley Strik
Nov 27 '18 at 22:27
1
@WesleyGroupshaveFeelingsToo Ah ok you are referring to that one $$ lim_{k rightarrow infty} frac{1}{2+frac{1}{k}}$$ That's fine!
– gimusi
Nov 27 '18 at 22:28
In my book a friendly introduction to analysis by Kosmala, it is referred to 2.2.1C
– Wesley Strik
Nov 27 '18 at 22:28
1
1
That first proof looks good, and is how I would solve this problem.
– Dave
Nov 27 '18 at 22:22
That first proof looks good, and is how I would solve this problem.
– Dave
Nov 27 '18 at 22:22
Both ways are fine, but what do you mean with "the fraction limit theorem"?
– gimusi
Nov 27 '18 at 22:25
Both ways are fine, but what do you mean with "the fraction limit theorem"?
– gimusi
Nov 27 '18 at 22:25
One of the standard results for limits is that if we take a limit of a fraction of two sequences, it is the same as the fraction of limits.
– Wesley Strik
Nov 27 '18 at 22:27
One of the standard results for limits is that if we take a limit of a fraction of two sequences, it is the same as the fraction of limits.
– Wesley Strik
Nov 27 '18 at 22:27
1
1
@WesleyGroupshaveFeelingsToo Ah ok you are referring to that one $$ lim_{k rightarrow infty} frac{1}{2+frac{1}{k}}$$ That's fine!
– gimusi
Nov 27 '18 at 22:28
@WesleyGroupshaveFeelingsToo Ah ok you are referring to that one $$ lim_{k rightarrow infty} frac{1}{2+frac{1}{k}}$$ That's fine!
– gimusi
Nov 27 '18 at 22:28
In my book a friendly introduction to analysis by Kosmala, it is referred to 2.2.1C
– Wesley Strik
Nov 27 '18 at 22:28
In my book a friendly introduction to analysis by Kosmala, it is referred to 2.2.1C
– Wesley Strik
Nov 27 '18 at 22:28
|
show 2 more comments
2 Answers
2
active
oldest
votes
Yes you are correct since $a_n not to 0$ the series does not converges and since it is with positive terms it diverges to $+infty$.
Also the alternative way is fine.
Is it fine because a sum is the limit of partial sums and for limits we know that a divergent limit to $infty$, times any positive constant still is a divergent limit?
– Wesley Strik
Nov 27 '18 at 22:37
in maths: ${ s_n } rightarrow infty $ then for $c>0$, also ${ c s_n } rightarrow infty $
– Wesley Strik
Nov 27 '18 at 22:38
@WesleyGroupshaveFeelingsToo Yes my observation is aimed to stress that form $a_n not to 0$ we can conclude in general that the series "does not converges" and in taht particular case it diverges to $infty$. I prefer use that distinction since we can also have the possibility of bounded but not convergent series.
– gimusi
Nov 27 '18 at 22:42
add a comment |
Or you can use the following $${kover 2k+1}ge {kover 3k}={1over 3}$$for $kge 1$ which implies a direct divergence. Still, your proof sounds right to me.
Too quick on getting rid of the initial $k$ haha.
– Wesley Strik
Nov 27 '18 at 22:39
Thanks, that was useful.
– Wesley Strik
Nov 27 '18 at 22:39
1
You're welcome. Good luck!
– Mostafa Ayaz
Nov 27 '18 at 22:40
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%2f3016384%2fdivergence-of-series-sum-frack2k1%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
Yes you are correct since $a_n not to 0$ the series does not converges and since it is with positive terms it diverges to $+infty$.
Also the alternative way is fine.
Is it fine because a sum is the limit of partial sums and for limits we know that a divergent limit to $infty$, times any positive constant still is a divergent limit?
– Wesley Strik
Nov 27 '18 at 22:37
in maths: ${ s_n } rightarrow infty $ then for $c>0$, also ${ c s_n } rightarrow infty $
– Wesley Strik
Nov 27 '18 at 22:38
@WesleyGroupshaveFeelingsToo Yes my observation is aimed to stress that form $a_n not to 0$ we can conclude in general that the series "does not converges" and in taht particular case it diverges to $infty$. I prefer use that distinction since we can also have the possibility of bounded but not convergent series.
– gimusi
Nov 27 '18 at 22:42
add a comment |
Yes you are correct since $a_n not to 0$ the series does not converges and since it is with positive terms it diverges to $+infty$.
Also the alternative way is fine.
Is it fine because a sum is the limit of partial sums and for limits we know that a divergent limit to $infty$, times any positive constant still is a divergent limit?
– Wesley Strik
Nov 27 '18 at 22:37
in maths: ${ s_n } rightarrow infty $ then for $c>0$, also ${ c s_n } rightarrow infty $
– Wesley Strik
Nov 27 '18 at 22:38
@WesleyGroupshaveFeelingsToo Yes my observation is aimed to stress that form $a_n not to 0$ we can conclude in general that the series "does not converges" and in taht particular case it diverges to $infty$. I prefer use that distinction since we can also have the possibility of bounded but not convergent series.
– gimusi
Nov 27 '18 at 22:42
add a comment |
Yes you are correct since $a_n not to 0$ the series does not converges and since it is with positive terms it diverges to $+infty$.
Also the alternative way is fine.
Yes you are correct since $a_n not to 0$ the series does not converges and since it is with positive terms it diverges to $+infty$.
Also the alternative way is fine.
answered Nov 27 '18 at 22:23
gimusigimusi
1
1
Is it fine because a sum is the limit of partial sums and for limits we know that a divergent limit to $infty$, times any positive constant still is a divergent limit?
– Wesley Strik
Nov 27 '18 at 22:37
in maths: ${ s_n } rightarrow infty $ then for $c>0$, also ${ c s_n } rightarrow infty $
– Wesley Strik
Nov 27 '18 at 22:38
@WesleyGroupshaveFeelingsToo Yes my observation is aimed to stress that form $a_n not to 0$ we can conclude in general that the series "does not converges" and in taht particular case it diverges to $infty$. I prefer use that distinction since we can also have the possibility of bounded but not convergent series.
– gimusi
Nov 27 '18 at 22:42
add a comment |
Is it fine because a sum is the limit of partial sums and for limits we know that a divergent limit to $infty$, times any positive constant still is a divergent limit?
– Wesley Strik
Nov 27 '18 at 22:37
in maths: ${ s_n } rightarrow infty $ then for $c>0$, also ${ c s_n } rightarrow infty $
– Wesley Strik
Nov 27 '18 at 22:38
@WesleyGroupshaveFeelingsToo Yes my observation is aimed to stress that form $a_n not to 0$ we can conclude in general that the series "does not converges" and in taht particular case it diverges to $infty$. I prefer use that distinction since we can also have the possibility of bounded but not convergent series.
– gimusi
Nov 27 '18 at 22:42
Is it fine because a sum is the limit of partial sums and for limits we know that a divergent limit to $infty$, times any positive constant still is a divergent limit?
– Wesley Strik
Nov 27 '18 at 22:37
Is it fine because a sum is the limit of partial sums and for limits we know that a divergent limit to $infty$, times any positive constant still is a divergent limit?
– Wesley Strik
Nov 27 '18 at 22:37
in maths: ${ s_n } rightarrow infty $ then for $c>0$, also ${ c s_n } rightarrow infty $
– Wesley Strik
Nov 27 '18 at 22:38
in maths: ${ s_n } rightarrow infty $ then for $c>0$, also ${ c s_n } rightarrow infty $
– Wesley Strik
Nov 27 '18 at 22:38
@WesleyGroupshaveFeelingsToo Yes my observation is aimed to stress that form $a_n not to 0$ we can conclude in general that the series "does not converges" and in taht particular case it diverges to $infty$. I prefer use that distinction since we can also have the possibility of bounded but not convergent series.
– gimusi
Nov 27 '18 at 22:42
@WesleyGroupshaveFeelingsToo Yes my observation is aimed to stress that form $a_n not to 0$ we can conclude in general that the series "does not converges" and in taht particular case it diverges to $infty$. I prefer use that distinction since we can also have the possibility of bounded but not convergent series.
– gimusi
Nov 27 '18 at 22:42
add a comment |
Or you can use the following $${kover 2k+1}ge {kover 3k}={1over 3}$$for $kge 1$ which implies a direct divergence. Still, your proof sounds right to me.
Too quick on getting rid of the initial $k$ haha.
– Wesley Strik
Nov 27 '18 at 22:39
Thanks, that was useful.
– Wesley Strik
Nov 27 '18 at 22:39
1
You're welcome. Good luck!
– Mostafa Ayaz
Nov 27 '18 at 22:40
add a comment |
Or you can use the following $${kover 2k+1}ge {kover 3k}={1over 3}$$for $kge 1$ which implies a direct divergence. Still, your proof sounds right to me.
Too quick on getting rid of the initial $k$ haha.
– Wesley Strik
Nov 27 '18 at 22:39
Thanks, that was useful.
– Wesley Strik
Nov 27 '18 at 22:39
1
You're welcome. Good luck!
– Mostafa Ayaz
Nov 27 '18 at 22:40
add a comment |
Or you can use the following $${kover 2k+1}ge {kover 3k}={1over 3}$$for $kge 1$ which implies a direct divergence. Still, your proof sounds right to me.
Or you can use the following $${kover 2k+1}ge {kover 3k}={1over 3}$$for $kge 1$ which implies a direct divergence. Still, your proof sounds right to me.
answered Nov 27 '18 at 22:37
Mostafa AyazMostafa Ayaz
14.5k3937
14.5k3937
Too quick on getting rid of the initial $k$ haha.
– Wesley Strik
Nov 27 '18 at 22:39
Thanks, that was useful.
– Wesley Strik
Nov 27 '18 at 22:39
1
You're welcome. Good luck!
– Mostafa Ayaz
Nov 27 '18 at 22:40
add a comment |
Too quick on getting rid of the initial $k$ haha.
– Wesley Strik
Nov 27 '18 at 22:39
Thanks, that was useful.
– Wesley Strik
Nov 27 '18 at 22:39
1
You're welcome. Good luck!
– Mostafa Ayaz
Nov 27 '18 at 22:40
Too quick on getting rid of the initial $k$ haha.
– Wesley Strik
Nov 27 '18 at 22:39
Too quick on getting rid of the initial $k$ haha.
– Wesley Strik
Nov 27 '18 at 22:39
Thanks, that was useful.
– Wesley Strik
Nov 27 '18 at 22:39
Thanks, that was useful.
– Wesley Strik
Nov 27 '18 at 22:39
1
1
You're welcome. Good luck!
– Mostafa Ayaz
Nov 27 '18 at 22:40
You're welcome. Good luck!
– Mostafa Ayaz
Nov 27 '18 at 22:40
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.
Some of your past answers have not been well-received, and you're in danger of being blocked from answering.
Please pay close attention to the following guidance:
- 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.
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%2f3016384%2fdivergence-of-series-sum-frack2k1%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
1
That first proof looks good, and is how I would solve this problem.
– Dave
Nov 27 '18 at 22:22
Both ways are fine, but what do you mean with "the fraction limit theorem"?
– gimusi
Nov 27 '18 at 22:25
One of the standard results for limits is that if we take a limit of a fraction of two sequences, it is the same as the fraction of limits.
– Wesley Strik
Nov 27 '18 at 22:27
1
@WesleyGroupshaveFeelingsToo Ah ok you are referring to that one $$ lim_{k rightarrow infty} frac{1}{2+frac{1}{k}}$$ That's fine!
– gimusi
Nov 27 '18 at 22:28
In my book a friendly introduction to analysis by Kosmala, it is referred to 2.2.1C
– Wesley Strik
Nov 27 '18 at 22:28