What value does $frac{1}{n} sqrt[n]{n(n+1)(n+2)cdots(2n)}$ tend to? [duplicate]
This question already has an answer here:
How to prove that $lim frac{1}{n} sqrt[n]{(n+1)(n+2)… 2n} = frac{4}{e}$
5 answers
I need to find where this sequence tends to:
$$frac{1}{n} sqrt[n]{n(n+1)(n+2)...(2n)}$$
My answer is $2$, by using a trick for the $n^{th}$ root: I take the inside and find where $dfrac{A_{n+1}}{A_n}$ goes, and this is where the original sequence goes.
sequences-and-series
marked as duplicate by Arnaud D., Martin Sleziak, Don Thousand, amWhy, José Carlos Santos Nov 21 '18 at 23:52
This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.
add a comment |
This question already has an answer here:
How to prove that $lim frac{1}{n} sqrt[n]{(n+1)(n+2)… 2n} = frac{4}{e}$
5 answers
I need to find where this sequence tends to:
$$frac{1}{n} sqrt[n]{n(n+1)(n+2)...(2n)}$$
My answer is $2$, by using a trick for the $n^{th}$ root: I take the inside and find where $dfrac{A_{n+1}}{A_n}$ goes, and this is where the original sequence goes.
sequences-and-series
marked as duplicate by Arnaud D., Martin Sleziak, Don Thousand, amWhy, José Carlos Santos Nov 21 '18 at 23:52
This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.
1
Which part of the expression is under the root? And which part is in the denominator?
– Yuriy S
Nov 10 '18 at 15:00
all of it after the root times the 1/n
– stelioball
Nov 10 '18 at 15:00
Please use the following code:frac{numerator}{denominator}
andsqrt[n]{expression}
– Yuriy S
Nov 10 '18 at 15:01
I’ve edited it. Use braces for roots, exponents, subscripts, etc. Also, using fractions is better.
– KM101
Nov 10 '18 at 15:01
3
As the $n$-th root is clearly $>n$, there's no way the sequence can tend to zero.
– Lord Shark the Unknown
Nov 10 '18 at 15:12
add a comment |
This question already has an answer here:
How to prove that $lim frac{1}{n} sqrt[n]{(n+1)(n+2)… 2n} = frac{4}{e}$
5 answers
I need to find where this sequence tends to:
$$frac{1}{n} sqrt[n]{n(n+1)(n+2)...(2n)}$$
My answer is $2$, by using a trick for the $n^{th}$ root: I take the inside and find where $dfrac{A_{n+1}}{A_n}$ goes, and this is where the original sequence goes.
sequences-and-series
This question already has an answer here:
How to prove that $lim frac{1}{n} sqrt[n]{(n+1)(n+2)… 2n} = frac{4}{e}$
5 answers
I need to find where this sequence tends to:
$$frac{1}{n} sqrt[n]{n(n+1)(n+2)...(2n)}$$
My answer is $2$, by using a trick for the $n^{th}$ root: I take the inside and find where $dfrac{A_{n+1}}{A_n}$ goes, and this is where the original sequence goes.
This question already has an answer here:
How to prove that $lim frac{1}{n} sqrt[n]{(n+1)(n+2)… 2n} = frac{4}{e}$
5 answers
sequences-and-series
sequences-and-series
edited Nov 11 '18 at 13:35
stelioball
asked Nov 10 '18 at 14:56
stelioballstelioball
355
355
marked as duplicate by Arnaud D., Martin Sleziak, Don Thousand, amWhy, José Carlos Santos Nov 21 '18 at 23:52
This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.
marked as duplicate by Arnaud D., Martin Sleziak, Don Thousand, amWhy, José Carlos Santos Nov 21 '18 at 23:52
This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.
1
Which part of the expression is under the root? And which part is in the denominator?
– Yuriy S
Nov 10 '18 at 15:00
all of it after the root times the 1/n
– stelioball
Nov 10 '18 at 15:00
Please use the following code:frac{numerator}{denominator}
andsqrt[n]{expression}
– Yuriy S
Nov 10 '18 at 15:01
I’ve edited it. Use braces for roots, exponents, subscripts, etc. Also, using fractions is better.
– KM101
Nov 10 '18 at 15:01
3
As the $n$-th root is clearly $>n$, there's no way the sequence can tend to zero.
– Lord Shark the Unknown
Nov 10 '18 at 15:12
add a comment |
1
Which part of the expression is under the root? And which part is in the denominator?
– Yuriy S
Nov 10 '18 at 15:00
all of it after the root times the 1/n
– stelioball
Nov 10 '18 at 15:00
Please use the following code:frac{numerator}{denominator}
andsqrt[n]{expression}
– Yuriy S
Nov 10 '18 at 15:01
I’ve edited it. Use braces for roots, exponents, subscripts, etc. Also, using fractions is better.
– KM101
Nov 10 '18 at 15:01
3
As the $n$-th root is clearly $>n$, there's no way the sequence can tend to zero.
– Lord Shark the Unknown
Nov 10 '18 at 15:12
1
1
Which part of the expression is under the root? And which part is in the denominator?
– Yuriy S
Nov 10 '18 at 15:00
Which part of the expression is under the root? And which part is in the denominator?
– Yuriy S
Nov 10 '18 at 15:00
all of it after the root times the 1/n
– stelioball
Nov 10 '18 at 15:00
all of it after the root times the 1/n
– stelioball
Nov 10 '18 at 15:00
Please use the following code:
frac{numerator}{denominator}
and sqrt[n]{expression}
– Yuriy S
Nov 10 '18 at 15:01
Please use the following code:
frac{numerator}{denominator}
and sqrt[n]{expression}
– Yuriy S
Nov 10 '18 at 15:01
I’ve edited it. Use braces for roots, exponents, subscripts, etc. Also, using fractions is better.
– KM101
Nov 10 '18 at 15:01
I’ve edited it. Use braces for roots, exponents, subscripts, etc. Also, using fractions is better.
– KM101
Nov 10 '18 at 15:01
3
3
As the $n$-th root is clearly $>n$, there's no way the sequence can tend to zero.
– Lord Shark the Unknown
Nov 10 '18 at 15:12
As the $n$-th root is clearly $>n$, there's no way the sequence can tend to zero.
– Lord Shark the Unknown
Nov 10 '18 at 15:12
add a comment |
3 Answers
3
active
oldest
votes
By ratio root criterion we have
$$a_n=frac{n(n+1)(n+2)...(2n)}{n^n} quad b_n=frac{1}{n} sqrt[n]{n(n+1)(n+2)...(2n)}=sqrt[n] a_n$$
then
$$frac{a_{n+1}}{a_n}=frac{(n+1)(n+2)...(2n+2)}{n(n+1)(n+2)...(2n)}frac{n^n}{(n+1)^{n+1}}=frac{(2n+2)(2n+1)}{n(n+1)}frac{1}{left(1+frac1nright)^n}to frac 4 e$$
which implies that $b_n to frac 4 e$.
1
Gimusi.Very nice!
– Peter Szilas
Nov 10 '18 at 17:08
Thanks my dear friend! :)
– gimusi
Nov 10 '18 at 17:21
add a comment |
$$ a_n
= frac{1}{n} sqrt[n]{n(n+1)(n+2)cdots (2n)}
= sqrt[n]{1(1+frac{1}{n})(1+frac{2}{n})cdots (1+frac{n}{n})}.
$$
$$
log a_n
= frac{1}{n} { log 1 + log (1+frac{1}{n}) + log (1+frac{2}{n})
+ cdots + log (1+frac{n}{n}) }.
$$
We know the above equations. Hence,
$$
limlimits_{n rightarrow infty} log a_n = int_1^2 log x dx
= [xlog x -x ]_1^2 = 2log 2 - 1 = log frac{4}{e}.
$$
Thus, the limit of $a_n$ is $frac{4}{e}$.
Are you using Riemann sum?
– gimusi
Nov 10 '18 at 15:27
@gimusi Yes, $log x$ is continuous, and so Riemann integrable.
– Doyun Nam
Nov 10 '18 at 15:33
Maybe you could explicitly refer to it to make it clear.
– gimusi
Nov 10 '18 at 16:17
add a comment |
By Stirling's formula,
begin{align}
lnleft[(n+1)(n+2)cdots(2n)right]
&=ln (2n)!-ln n!=(2n+1/2)ln(2n)-(n+1/2)ln n-n+o(1)\
&=(2n+1/2)ln 2+nln n-n+o(1)
end{align}
and so
$$lnleft[n(n+1)(n+2)cdots(2n)right]
=(2n+1/2)ln 2+(n+1)ln n-n+o(1).$$
Then
$$lnsqrt[n]{n(n+1)(n+2)cdots(2n)}=2ln 2+ln n-1+o(1)$$
and
$$lnfrac{sqrt[n]{n(n+1)(n+2)cdots(2n)}}n=2ln 2-1+o(1).$$
We conclude that your sequence converges to $4/e$.
add a comment |
3 Answers
3
active
oldest
votes
3 Answers
3
active
oldest
votes
active
oldest
votes
active
oldest
votes
By ratio root criterion we have
$$a_n=frac{n(n+1)(n+2)...(2n)}{n^n} quad b_n=frac{1}{n} sqrt[n]{n(n+1)(n+2)...(2n)}=sqrt[n] a_n$$
then
$$frac{a_{n+1}}{a_n}=frac{(n+1)(n+2)...(2n+2)}{n(n+1)(n+2)...(2n)}frac{n^n}{(n+1)^{n+1}}=frac{(2n+2)(2n+1)}{n(n+1)}frac{1}{left(1+frac1nright)^n}to frac 4 e$$
which implies that $b_n to frac 4 e$.
1
Gimusi.Very nice!
– Peter Szilas
Nov 10 '18 at 17:08
Thanks my dear friend! :)
– gimusi
Nov 10 '18 at 17:21
add a comment |
By ratio root criterion we have
$$a_n=frac{n(n+1)(n+2)...(2n)}{n^n} quad b_n=frac{1}{n} sqrt[n]{n(n+1)(n+2)...(2n)}=sqrt[n] a_n$$
then
$$frac{a_{n+1}}{a_n}=frac{(n+1)(n+2)...(2n+2)}{n(n+1)(n+2)...(2n)}frac{n^n}{(n+1)^{n+1}}=frac{(2n+2)(2n+1)}{n(n+1)}frac{1}{left(1+frac1nright)^n}to frac 4 e$$
which implies that $b_n to frac 4 e$.
1
Gimusi.Very nice!
– Peter Szilas
Nov 10 '18 at 17:08
Thanks my dear friend! :)
– gimusi
Nov 10 '18 at 17:21
add a comment |
By ratio root criterion we have
$$a_n=frac{n(n+1)(n+2)...(2n)}{n^n} quad b_n=frac{1}{n} sqrt[n]{n(n+1)(n+2)...(2n)}=sqrt[n] a_n$$
then
$$frac{a_{n+1}}{a_n}=frac{(n+1)(n+2)...(2n+2)}{n(n+1)(n+2)...(2n)}frac{n^n}{(n+1)^{n+1}}=frac{(2n+2)(2n+1)}{n(n+1)}frac{1}{left(1+frac1nright)^n}to frac 4 e$$
which implies that $b_n to frac 4 e$.
By ratio root criterion we have
$$a_n=frac{n(n+1)(n+2)...(2n)}{n^n} quad b_n=frac{1}{n} sqrt[n]{n(n+1)(n+2)...(2n)}=sqrt[n] a_n$$
then
$$frac{a_{n+1}}{a_n}=frac{(n+1)(n+2)...(2n+2)}{n(n+1)(n+2)...(2n)}frac{n^n}{(n+1)^{n+1}}=frac{(2n+2)(2n+1)}{n(n+1)}frac{1}{left(1+frac1nright)^n}to frac 4 e$$
which implies that $b_n to frac 4 e$.
edited Nov 21 '18 at 18:01
answered Nov 10 '18 at 15:20


gimusigimusi
1
1
1
Gimusi.Very nice!
– Peter Szilas
Nov 10 '18 at 17:08
Thanks my dear friend! :)
– gimusi
Nov 10 '18 at 17:21
add a comment |
1
Gimusi.Very nice!
– Peter Szilas
Nov 10 '18 at 17:08
Thanks my dear friend! :)
– gimusi
Nov 10 '18 at 17:21
1
1
Gimusi.Very nice!
– Peter Szilas
Nov 10 '18 at 17:08
Gimusi.Very nice!
– Peter Szilas
Nov 10 '18 at 17:08
Thanks my dear friend! :)
– gimusi
Nov 10 '18 at 17:21
Thanks my dear friend! :)
– gimusi
Nov 10 '18 at 17:21
add a comment |
$$ a_n
= frac{1}{n} sqrt[n]{n(n+1)(n+2)cdots (2n)}
= sqrt[n]{1(1+frac{1}{n})(1+frac{2}{n})cdots (1+frac{n}{n})}.
$$
$$
log a_n
= frac{1}{n} { log 1 + log (1+frac{1}{n}) + log (1+frac{2}{n})
+ cdots + log (1+frac{n}{n}) }.
$$
We know the above equations. Hence,
$$
limlimits_{n rightarrow infty} log a_n = int_1^2 log x dx
= [xlog x -x ]_1^2 = 2log 2 - 1 = log frac{4}{e}.
$$
Thus, the limit of $a_n$ is $frac{4}{e}$.
Are you using Riemann sum?
– gimusi
Nov 10 '18 at 15:27
@gimusi Yes, $log x$ is continuous, and so Riemann integrable.
– Doyun Nam
Nov 10 '18 at 15:33
Maybe you could explicitly refer to it to make it clear.
– gimusi
Nov 10 '18 at 16:17
add a comment |
$$ a_n
= frac{1}{n} sqrt[n]{n(n+1)(n+2)cdots (2n)}
= sqrt[n]{1(1+frac{1}{n})(1+frac{2}{n})cdots (1+frac{n}{n})}.
$$
$$
log a_n
= frac{1}{n} { log 1 + log (1+frac{1}{n}) + log (1+frac{2}{n})
+ cdots + log (1+frac{n}{n}) }.
$$
We know the above equations. Hence,
$$
limlimits_{n rightarrow infty} log a_n = int_1^2 log x dx
= [xlog x -x ]_1^2 = 2log 2 - 1 = log frac{4}{e}.
$$
Thus, the limit of $a_n$ is $frac{4}{e}$.
Are you using Riemann sum?
– gimusi
Nov 10 '18 at 15:27
@gimusi Yes, $log x$ is continuous, and so Riemann integrable.
– Doyun Nam
Nov 10 '18 at 15:33
Maybe you could explicitly refer to it to make it clear.
– gimusi
Nov 10 '18 at 16:17
add a comment |
$$ a_n
= frac{1}{n} sqrt[n]{n(n+1)(n+2)cdots (2n)}
= sqrt[n]{1(1+frac{1}{n})(1+frac{2}{n})cdots (1+frac{n}{n})}.
$$
$$
log a_n
= frac{1}{n} { log 1 + log (1+frac{1}{n}) + log (1+frac{2}{n})
+ cdots + log (1+frac{n}{n}) }.
$$
We know the above equations. Hence,
$$
limlimits_{n rightarrow infty} log a_n = int_1^2 log x dx
= [xlog x -x ]_1^2 = 2log 2 - 1 = log frac{4}{e}.
$$
Thus, the limit of $a_n$ is $frac{4}{e}$.
$$ a_n
= frac{1}{n} sqrt[n]{n(n+1)(n+2)cdots (2n)}
= sqrt[n]{1(1+frac{1}{n})(1+frac{2}{n})cdots (1+frac{n}{n})}.
$$
$$
log a_n
= frac{1}{n} { log 1 + log (1+frac{1}{n}) + log (1+frac{2}{n})
+ cdots + log (1+frac{n}{n}) }.
$$
We know the above equations. Hence,
$$
limlimits_{n rightarrow infty} log a_n = int_1^2 log x dx
= [xlog x -x ]_1^2 = 2log 2 - 1 = log frac{4}{e}.
$$
Thus, the limit of $a_n$ is $frac{4}{e}$.
answered Nov 10 '18 at 15:17


Doyun NamDoyun Nam
41619
41619
Are you using Riemann sum?
– gimusi
Nov 10 '18 at 15:27
@gimusi Yes, $log x$ is continuous, and so Riemann integrable.
– Doyun Nam
Nov 10 '18 at 15:33
Maybe you could explicitly refer to it to make it clear.
– gimusi
Nov 10 '18 at 16:17
add a comment |
Are you using Riemann sum?
– gimusi
Nov 10 '18 at 15:27
@gimusi Yes, $log x$ is continuous, and so Riemann integrable.
– Doyun Nam
Nov 10 '18 at 15:33
Maybe you could explicitly refer to it to make it clear.
– gimusi
Nov 10 '18 at 16:17
Are you using Riemann sum?
– gimusi
Nov 10 '18 at 15:27
Are you using Riemann sum?
– gimusi
Nov 10 '18 at 15:27
@gimusi Yes, $log x$ is continuous, and so Riemann integrable.
– Doyun Nam
Nov 10 '18 at 15:33
@gimusi Yes, $log x$ is continuous, and so Riemann integrable.
– Doyun Nam
Nov 10 '18 at 15:33
Maybe you could explicitly refer to it to make it clear.
– gimusi
Nov 10 '18 at 16:17
Maybe you could explicitly refer to it to make it clear.
– gimusi
Nov 10 '18 at 16:17
add a comment |
By Stirling's formula,
begin{align}
lnleft[(n+1)(n+2)cdots(2n)right]
&=ln (2n)!-ln n!=(2n+1/2)ln(2n)-(n+1/2)ln n-n+o(1)\
&=(2n+1/2)ln 2+nln n-n+o(1)
end{align}
and so
$$lnleft[n(n+1)(n+2)cdots(2n)right]
=(2n+1/2)ln 2+(n+1)ln n-n+o(1).$$
Then
$$lnsqrt[n]{n(n+1)(n+2)cdots(2n)}=2ln 2+ln n-1+o(1)$$
and
$$lnfrac{sqrt[n]{n(n+1)(n+2)cdots(2n)}}n=2ln 2-1+o(1).$$
We conclude that your sequence converges to $4/e$.
add a comment |
By Stirling's formula,
begin{align}
lnleft[(n+1)(n+2)cdots(2n)right]
&=ln (2n)!-ln n!=(2n+1/2)ln(2n)-(n+1/2)ln n-n+o(1)\
&=(2n+1/2)ln 2+nln n-n+o(1)
end{align}
and so
$$lnleft[n(n+1)(n+2)cdots(2n)right]
=(2n+1/2)ln 2+(n+1)ln n-n+o(1).$$
Then
$$lnsqrt[n]{n(n+1)(n+2)cdots(2n)}=2ln 2+ln n-1+o(1)$$
and
$$lnfrac{sqrt[n]{n(n+1)(n+2)cdots(2n)}}n=2ln 2-1+o(1).$$
We conclude that your sequence converges to $4/e$.
add a comment |
By Stirling's formula,
begin{align}
lnleft[(n+1)(n+2)cdots(2n)right]
&=ln (2n)!-ln n!=(2n+1/2)ln(2n)-(n+1/2)ln n-n+o(1)\
&=(2n+1/2)ln 2+nln n-n+o(1)
end{align}
and so
$$lnleft[n(n+1)(n+2)cdots(2n)right]
=(2n+1/2)ln 2+(n+1)ln n-n+o(1).$$
Then
$$lnsqrt[n]{n(n+1)(n+2)cdots(2n)}=2ln 2+ln n-1+o(1)$$
and
$$lnfrac{sqrt[n]{n(n+1)(n+2)cdots(2n)}}n=2ln 2-1+o(1).$$
We conclude that your sequence converges to $4/e$.
By Stirling's formula,
begin{align}
lnleft[(n+1)(n+2)cdots(2n)right]
&=ln (2n)!-ln n!=(2n+1/2)ln(2n)-(n+1/2)ln n-n+o(1)\
&=(2n+1/2)ln 2+nln n-n+o(1)
end{align}
and so
$$lnleft[n(n+1)(n+2)cdots(2n)right]
=(2n+1/2)ln 2+(n+1)ln n-n+o(1).$$
Then
$$lnsqrt[n]{n(n+1)(n+2)cdots(2n)}=2ln 2+ln n-1+o(1)$$
and
$$lnfrac{sqrt[n]{n(n+1)(n+2)cdots(2n)}}n=2ln 2-1+o(1).$$
We conclude that your sequence converges to $4/e$.
answered Nov 10 '18 at 15:17
Lord Shark the UnknownLord Shark the Unknown
102k959132
102k959132
add a comment |
add a comment |
1
Which part of the expression is under the root? And which part is in the denominator?
– Yuriy S
Nov 10 '18 at 15:00
all of it after the root times the 1/n
– stelioball
Nov 10 '18 at 15:00
Please use the following code:
frac{numerator}{denominator}
andsqrt[n]{expression}
– Yuriy S
Nov 10 '18 at 15:01
I’ve edited it. Use braces for roots, exponents, subscripts, etc. Also, using fractions is better.
– KM101
Nov 10 '18 at 15:01
3
As the $n$-th root is clearly $>n$, there's no way the sequence can tend to zero.
– Lord Shark the Unknown
Nov 10 '18 at 15:12