If ${a_n}$ is bounded above and $a_{n+1} - a_{n} > -frac{1}{n^{2}}$, then is ${a_n}$ convergent?
$begingroup$
Suppose that a sequence ${a_n}$ is bounded above and if satisfies the condition $a_{n+1} - a_{n} > frac{-1}{n^{2}},$ where $n$ is a natural number. Then ${a_n}$ is convergent. $($true or false$)?$
$a_{n+1} - a_{n} > frac{-1}{n^{2}}$
$a_n - a_{n+1} < frac{1}{n^{2}}$
So, this sequence is monotonically decreasing.
And $$lim_{n to infty} (a_n - a_{n-1})= 0$$
How to proceed to see, if the sequence converges$?$
real-analysis
$endgroup$
add a comment |
$begingroup$
Suppose that a sequence ${a_n}$ is bounded above and if satisfies the condition $a_{n+1} - a_{n} > frac{-1}{n^{2}},$ where $n$ is a natural number. Then ${a_n}$ is convergent. $($true or false$)?$
$a_{n+1} - a_{n} > frac{-1}{n^{2}}$
$a_n - a_{n+1} < frac{1}{n^{2}}$
So, this sequence is monotonically decreasing.
And $$lim_{n to infty} (a_n - a_{n-1})= 0$$
How to proceed to see, if the sequence converges$?$
real-analysis
$endgroup$
$begingroup$
It's not necessarily monitonically decreasing. There is nothing in the conditions to imply that $a_{n+1}-a_n$ can't be greater than $0$.
$endgroup$
– TonyK
Dec 27 '18 at 14:35
add a comment |
$begingroup$
Suppose that a sequence ${a_n}$ is bounded above and if satisfies the condition $a_{n+1} - a_{n} > frac{-1}{n^{2}},$ where $n$ is a natural number. Then ${a_n}$ is convergent. $($true or false$)?$
$a_{n+1} - a_{n} > frac{-1}{n^{2}}$
$a_n - a_{n+1} < frac{1}{n^{2}}$
So, this sequence is monotonically decreasing.
And $$lim_{n to infty} (a_n - a_{n-1})= 0$$
How to proceed to see, if the sequence converges$?$
real-analysis
$endgroup$
Suppose that a sequence ${a_n}$ is bounded above and if satisfies the condition $a_{n+1} - a_{n} > frac{-1}{n^{2}},$ where $n$ is a natural number. Then ${a_n}$ is convergent. $($true or false$)?$
$a_{n+1} - a_{n} > frac{-1}{n^{2}}$
$a_n - a_{n+1} < frac{1}{n^{2}}$
So, this sequence is monotonically decreasing.
And $$lim_{n to infty} (a_n - a_{n-1})= 0$$
How to proceed to see, if the sequence converges$?$
real-analysis
real-analysis
edited Dec 27 '18 at 13:29
StubbornAtom
6,17811339
6,17811339
asked Dec 27 '18 at 13:24
MathsaddictMathsaddict
3669
3669
$begingroup$
It's not necessarily monitonically decreasing. There is nothing in the conditions to imply that $a_{n+1}-a_n$ can't be greater than $0$.
$endgroup$
– TonyK
Dec 27 '18 at 14:35
add a comment |
$begingroup$
It's not necessarily monitonically decreasing. There is nothing in the conditions to imply that $a_{n+1}-a_n$ can't be greater than $0$.
$endgroup$
– TonyK
Dec 27 '18 at 14:35
$begingroup$
It's not necessarily monitonically decreasing. There is nothing in the conditions to imply that $a_{n+1}-a_n$ can't be greater than $0$.
$endgroup$
– TonyK
Dec 27 '18 at 14:35
$begingroup$
It's not necessarily monitonically decreasing. There is nothing in the conditions to imply that $a_{n+1}-a_n$ can't be greater than $0$.
$endgroup$
– TonyK
Dec 27 '18 at 14:35
add a comment |
2 Answers
2
active
oldest
votes
$begingroup$
$$frac{1}{n^2} = b_n-b_{n-1}, ngeq2, b_1 = 1 $$
then
$$b_n = sum_{k=1}^n frac{1}{k^2} $$
Then the following sequence is convergent, because it's increasing and bounded:
$$x_n = b_n+a_n $$
Hence $a_n = x_n-b_n$ is also convergent.
$endgroup$
add a comment |
$begingroup$
First of all $a_{n}-a_{n+1}<frac{1}{n^{2}}$ doesn't mean the sequence is monotonically decreasing, on the contrary means that tends to be monotonically increasing.
For example the sequence $a_{n}=n$ satisfy the inequality.
In fact $a_{n}-a_{n+1}< 0$ means monotonically increasing and $a_{n}-a_{n+1}>0$ decreasing.
The proof of your question is that: the series $1/n^{2}$ converges, so $forallvarepsilon>0,exists n_{varepsilon}$ tc$forall N>n_{varepsilon}$ $sum_{n=N}^{infty}1/n^{2}<varepsilon$.
Because the sequence is upper bounded we can call $M:=limsup_{ntoinfty}a_{n}$, and we have a subsequence $a_{n_{k}}$ which converges to M. So $forallvarepsilon>0$ $exists k_{varepsilon}$ tc $|M-a_{n_{k_{varepsilon}}}|<varepsilon$.
From the hypotesis we know that if $n>n_{k}$ then $a_{n_{k}}-a_{n}<sum_{i=n_{k}}^{n}1/i^{2}<sum_{i=n_{k}}^{infty}1/i^{2}$, so $a_{n}>a_{n_{k}}-sum_{i=n_{k}}^{infty}1/i^{2}$.
It follows that for $k>max{k_{varepsilon},n_{varepsilon}}$ $a_{n}>M-2varepsilon$ $forall n>n_k$.
On the other hand let $M_{k}:=sup_{n>n_k}{a_{n}}$, so we have that obviously $a_{n}<M_{k}$ $forall n>k$, and it's clear because of the definition that $M_{k}to M$, and it does it decreasingly.
So for $k>m_{varepsilon}$ we have $M_{k}<M+varepsilon$.
So for $k>max{k_{varepsilon},n_{varepsilon}, m_{varepsilon}}$ and for $n>n_{k}$ we have $M+varepsilon>a_{n}>M-2varepsilon$, that means $a_nto M$.
$endgroup$
$begingroup$
Where does $|a_{n_{k_varepsilon}}-a_n|<sum_{i=k_varepsilon}^n frac{1}{i^2}$ comes from?
$endgroup$
– Jakobian
Dec 27 '18 at 15:21
$begingroup$
You are right, i've changed my answer, hope it's right now
$endgroup$
– ecrin
Dec 27 '18 at 16:55
$begingroup$
I think it's ok
$endgroup$
– Jakobian
Dec 27 '18 at 17:49
add a comment |
Your Answer
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2 Answers
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2 Answers
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$begingroup$
$$frac{1}{n^2} = b_n-b_{n-1}, ngeq2, b_1 = 1 $$
then
$$b_n = sum_{k=1}^n frac{1}{k^2} $$
Then the following sequence is convergent, because it's increasing and bounded:
$$x_n = b_n+a_n $$
Hence $a_n = x_n-b_n$ is also convergent.
$endgroup$
add a comment |
$begingroup$
$$frac{1}{n^2} = b_n-b_{n-1}, ngeq2, b_1 = 1 $$
then
$$b_n = sum_{k=1}^n frac{1}{k^2} $$
Then the following sequence is convergent, because it's increasing and bounded:
$$x_n = b_n+a_n $$
Hence $a_n = x_n-b_n$ is also convergent.
$endgroup$
add a comment |
$begingroup$
$$frac{1}{n^2} = b_n-b_{n-1}, ngeq2, b_1 = 1 $$
then
$$b_n = sum_{k=1}^n frac{1}{k^2} $$
Then the following sequence is convergent, because it's increasing and bounded:
$$x_n = b_n+a_n $$
Hence $a_n = x_n-b_n$ is also convergent.
$endgroup$
$$frac{1}{n^2} = b_n-b_{n-1}, ngeq2, b_1 = 1 $$
then
$$b_n = sum_{k=1}^n frac{1}{k^2} $$
Then the following sequence is convergent, because it's increasing and bounded:
$$x_n = b_n+a_n $$
Hence $a_n = x_n-b_n$ is also convergent.
answered Dec 27 '18 at 13:37
JakobianJakobian
2,695721
2,695721
add a comment |
add a comment |
$begingroup$
First of all $a_{n}-a_{n+1}<frac{1}{n^{2}}$ doesn't mean the sequence is monotonically decreasing, on the contrary means that tends to be monotonically increasing.
For example the sequence $a_{n}=n$ satisfy the inequality.
In fact $a_{n}-a_{n+1}< 0$ means monotonically increasing and $a_{n}-a_{n+1}>0$ decreasing.
The proof of your question is that: the series $1/n^{2}$ converges, so $forallvarepsilon>0,exists n_{varepsilon}$ tc$forall N>n_{varepsilon}$ $sum_{n=N}^{infty}1/n^{2}<varepsilon$.
Because the sequence is upper bounded we can call $M:=limsup_{ntoinfty}a_{n}$, and we have a subsequence $a_{n_{k}}$ which converges to M. So $forallvarepsilon>0$ $exists k_{varepsilon}$ tc $|M-a_{n_{k_{varepsilon}}}|<varepsilon$.
From the hypotesis we know that if $n>n_{k}$ then $a_{n_{k}}-a_{n}<sum_{i=n_{k}}^{n}1/i^{2}<sum_{i=n_{k}}^{infty}1/i^{2}$, so $a_{n}>a_{n_{k}}-sum_{i=n_{k}}^{infty}1/i^{2}$.
It follows that for $k>max{k_{varepsilon},n_{varepsilon}}$ $a_{n}>M-2varepsilon$ $forall n>n_k$.
On the other hand let $M_{k}:=sup_{n>n_k}{a_{n}}$, so we have that obviously $a_{n}<M_{k}$ $forall n>k$, and it's clear because of the definition that $M_{k}to M$, and it does it decreasingly.
So for $k>m_{varepsilon}$ we have $M_{k}<M+varepsilon$.
So for $k>max{k_{varepsilon},n_{varepsilon}, m_{varepsilon}}$ and for $n>n_{k}$ we have $M+varepsilon>a_{n}>M-2varepsilon$, that means $a_nto M$.
$endgroup$
$begingroup$
Where does $|a_{n_{k_varepsilon}}-a_n|<sum_{i=k_varepsilon}^n frac{1}{i^2}$ comes from?
$endgroup$
– Jakobian
Dec 27 '18 at 15:21
$begingroup$
You are right, i've changed my answer, hope it's right now
$endgroup$
– ecrin
Dec 27 '18 at 16:55
$begingroup$
I think it's ok
$endgroup$
– Jakobian
Dec 27 '18 at 17:49
add a comment |
$begingroup$
First of all $a_{n}-a_{n+1}<frac{1}{n^{2}}$ doesn't mean the sequence is monotonically decreasing, on the contrary means that tends to be monotonically increasing.
For example the sequence $a_{n}=n$ satisfy the inequality.
In fact $a_{n}-a_{n+1}< 0$ means monotonically increasing and $a_{n}-a_{n+1}>0$ decreasing.
The proof of your question is that: the series $1/n^{2}$ converges, so $forallvarepsilon>0,exists n_{varepsilon}$ tc$forall N>n_{varepsilon}$ $sum_{n=N}^{infty}1/n^{2}<varepsilon$.
Because the sequence is upper bounded we can call $M:=limsup_{ntoinfty}a_{n}$, and we have a subsequence $a_{n_{k}}$ which converges to M. So $forallvarepsilon>0$ $exists k_{varepsilon}$ tc $|M-a_{n_{k_{varepsilon}}}|<varepsilon$.
From the hypotesis we know that if $n>n_{k}$ then $a_{n_{k}}-a_{n}<sum_{i=n_{k}}^{n}1/i^{2}<sum_{i=n_{k}}^{infty}1/i^{2}$, so $a_{n}>a_{n_{k}}-sum_{i=n_{k}}^{infty}1/i^{2}$.
It follows that for $k>max{k_{varepsilon},n_{varepsilon}}$ $a_{n}>M-2varepsilon$ $forall n>n_k$.
On the other hand let $M_{k}:=sup_{n>n_k}{a_{n}}$, so we have that obviously $a_{n}<M_{k}$ $forall n>k$, and it's clear because of the definition that $M_{k}to M$, and it does it decreasingly.
So for $k>m_{varepsilon}$ we have $M_{k}<M+varepsilon$.
So for $k>max{k_{varepsilon},n_{varepsilon}, m_{varepsilon}}$ and for $n>n_{k}$ we have $M+varepsilon>a_{n}>M-2varepsilon$, that means $a_nto M$.
$endgroup$
$begingroup$
Where does $|a_{n_{k_varepsilon}}-a_n|<sum_{i=k_varepsilon}^n frac{1}{i^2}$ comes from?
$endgroup$
– Jakobian
Dec 27 '18 at 15:21
$begingroup$
You are right, i've changed my answer, hope it's right now
$endgroup$
– ecrin
Dec 27 '18 at 16:55
$begingroup$
I think it's ok
$endgroup$
– Jakobian
Dec 27 '18 at 17:49
add a comment |
$begingroup$
First of all $a_{n}-a_{n+1}<frac{1}{n^{2}}$ doesn't mean the sequence is monotonically decreasing, on the contrary means that tends to be monotonically increasing.
For example the sequence $a_{n}=n$ satisfy the inequality.
In fact $a_{n}-a_{n+1}< 0$ means monotonically increasing and $a_{n}-a_{n+1}>0$ decreasing.
The proof of your question is that: the series $1/n^{2}$ converges, so $forallvarepsilon>0,exists n_{varepsilon}$ tc$forall N>n_{varepsilon}$ $sum_{n=N}^{infty}1/n^{2}<varepsilon$.
Because the sequence is upper bounded we can call $M:=limsup_{ntoinfty}a_{n}$, and we have a subsequence $a_{n_{k}}$ which converges to M. So $forallvarepsilon>0$ $exists k_{varepsilon}$ tc $|M-a_{n_{k_{varepsilon}}}|<varepsilon$.
From the hypotesis we know that if $n>n_{k}$ then $a_{n_{k}}-a_{n}<sum_{i=n_{k}}^{n}1/i^{2}<sum_{i=n_{k}}^{infty}1/i^{2}$, so $a_{n}>a_{n_{k}}-sum_{i=n_{k}}^{infty}1/i^{2}$.
It follows that for $k>max{k_{varepsilon},n_{varepsilon}}$ $a_{n}>M-2varepsilon$ $forall n>n_k$.
On the other hand let $M_{k}:=sup_{n>n_k}{a_{n}}$, so we have that obviously $a_{n}<M_{k}$ $forall n>k$, and it's clear because of the definition that $M_{k}to M$, and it does it decreasingly.
So for $k>m_{varepsilon}$ we have $M_{k}<M+varepsilon$.
So for $k>max{k_{varepsilon},n_{varepsilon}, m_{varepsilon}}$ and for $n>n_{k}$ we have $M+varepsilon>a_{n}>M-2varepsilon$, that means $a_nto M$.
$endgroup$
First of all $a_{n}-a_{n+1}<frac{1}{n^{2}}$ doesn't mean the sequence is monotonically decreasing, on the contrary means that tends to be monotonically increasing.
For example the sequence $a_{n}=n$ satisfy the inequality.
In fact $a_{n}-a_{n+1}< 0$ means monotonically increasing and $a_{n}-a_{n+1}>0$ decreasing.
The proof of your question is that: the series $1/n^{2}$ converges, so $forallvarepsilon>0,exists n_{varepsilon}$ tc$forall N>n_{varepsilon}$ $sum_{n=N}^{infty}1/n^{2}<varepsilon$.
Because the sequence is upper bounded we can call $M:=limsup_{ntoinfty}a_{n}$, and we have a subsequence $a_{n_{k}}$ which converges to M. So $forallvarepsilon>0$ $exists k_{varepsilon}$ tc $|M-a_{n_{k_{varepsilon}}}|<varepsilon$.
From the hypotesis we know that if $n>n_{k}$ then $a_{n_{k}}-a_{n}<sum_{i=n_{k}}^{n}1/i^{2}<sum_{i=n_{k}}^{infty}1/i^{2}$, so $a_{n}>a_{n_{k}}-sum_{i=n_{k}}^{infty}1/i^{2}$.
It follows that for $k>max{k_{varepsilon},n_{varepsilon}}$ $a_{n}>M-2varepsilon$ $forall n>n_k$.
On the other hand let $M_{k}:=sup_{n>n_k}{a_{n}}$, so we have that obviously $a_{n}<M_{k}$ $forall n>k$, and it's clear because of the definition that $M_{k}to M$, and it does it decreasingly.
So for $k>m_{varepsilon}$ we have $M_{k}<M+varepsilon$.
So for $k>max{k_{varepsilon},n_{varepsilon}, m_{varepsilon}}$ and for $n>n_{k}$ we have $M+varepsilon>a_{n}>M-2varepsilon$, that means $a_nto M$.
edited Dec 27 '18 at 17:18
answered Dec 27 '18 at 15:00
ecrinecrin
3477
3477
$begingroup$
Where does $|a_{n_{k_varepsilon}}-a_n|<sum_{i=k_varepsilon}^n frac{1}{i^2}$ comes from?
$endgroup$
– Jakobian
Dec 27 '18 at 15:21
$begingroup$
You are right, i've changed my answer, hope it's right now
$endgroup$
– ecrin
Dec 27 '18 at 16:55
$begingroup$
I think it's ok
$endgroup$
– Jakobian
Dec 27 '18 at 17:49
add a comment |
$begingroup$
Where does $|a_{n_{k_varepsilon}}-a_n|<sum_{i=k_varepsilon}^n frac{1}{i^2}$ comes from?
$endgroup$
– Jakobian
Dec 27 '18 at 15:21
$begingroup$
You are right, i've changed my answer, hope it's right now
$endgroup$
– ecrin
Dec 27 '18 at 16:55
$begingroup$
I think it's ok
$endgroup$
– Jakobian
Dec 27 '18 at 17:49
$begingroup$
Where does $|a_{n_{k_varepsilon}}-a_n|<sum_{i=k_varepsilon}^n frac{1}{i^2}$ comes from?
$endgroup$
– Jakobian
Dec 27 '18 at 15:21
$begingroup$
Where does $|a_{n_{k_varepsilon}}-a_n|<sum_{i=k_varepsilon}^n frac{1}{i^2}$ comes from?
$endgroup$
– Jakobian
Dec 27 '18 at 15:21
$begingroup$
You are right, i've changed my answer, hope it's right now
$endgroup$
– ecrin
Dec 27 '18 at 16:55
$begingroup$
You are right, i've changed my answer, hope it's right now
$endgroup$
– ecrin
Dec 27 '18 at 16:55
$begingroup$
I think it's ok
$endgroup$
– Jakobian
Dec 27 '18 at 17:49
$begingroup$
I think it's ok
$endgroup$
– Jakobian
Dec 27 '18 at 17:49
add a comment |
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$begingroup$
It's not necessarily monitonically decreasing. There is nothing in the conditions to imply that $a_{n+1}-a_n$ can't be greater than $0$.
$endgroup$
– TonyK
Dec 27 '18 at 14:35