Does this series converges to some constant? [closed]
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Let $S = sum_i=1^log n fraci2^i$
Question : What is $S$ when $n$ tends to infinity? Is it a constant or some function of $n$?
sequences-and-series divergent-series
closed as unclear what you're asking by Jam, Simply Beautiful Art, user 108128, RRL, Key Flex Aug 2 at 14:29
Please clarify your specific problem or add additional details to highlight exactly what you need. As it's currently written, itâÂÂs hard to tell exactly what you're asking. See the How to Ask page for help clarifying this question. If this question can be reworded to fit the rules in the help center, please edit the question.
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Let $S = sum_i=1^log n fraci2^i$
Question : What is $S$ when $n$ tends to infinity? Is it a constant or some function of $n$?
sequences-and-series divergent-series
closed as unclear what you're asking by Jam, Simply Beautiful Art, user 108128, RRL, Key Flex Aug 2 at 14:29
Please clarify your specific problem or add additional details to highlight exactly what you need. As it's currently written, itâÂÂs hard to tell exactly what you're asking. See the How to Ask page for help clarifying this question. If this question can be reworded to fit the rules in the help center, please edit the question.
To me it appears to be something like $O(fraclognn)$.
â old
Aug 2 at 12:01
3
If we let $n$ tend to infinity, the resulting value can't really depend on $n$.
â Arthur
Aug 2 at 12:01
2
The upper limit should be a natural number. Do you mean perhaps $lfloor log nrfloor$ ?
â Peter
Aug 2 at 12:01
1
I am fine with Order notation also.
â lovw
Aug 2 at 12:01
3
I think the OP is not looking for the limit per se, but the asymptotic growth of $S(n)$, i.e. with respect to $n$.
â Eff
Aug 2 at 12:05
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up vote
0
down vote
favorite
up vote
0
down vote
favorite
Let $S = sum_i=1^log n fraci2^i$
Question : What is $S$ when $n$ tends to infinity? Is it a constant or some function of $n$?
sequences-and-series divergent-series
Let $S = sum_i=1^log n fraci2^i$
Question : What is $S$ when $n$ tends to infinity? Is it a constant or some function of $n$?
sequences-and-series divergent-series
edited Aug 2 at 12:34
BCLC
6,98221973
6,98221973
asked Aug 2 at 11:58
lovw
224
224
closed as unclear what you're asking by Jam, Simply Beautiful Art, user 108128, RRL, Key Flex Aug 2 at 14:29
Please clarify your specific problem or add additional details to highlight exactly what you need. As it's currently written, itâÂÂs hard to tell exactly what you're asking. See the How to Ask page for help clarifying this question. If this question can be reworded to fit the rules in the help center, please edit the question.
closed as unclear what you're asking by Jam, Simply Beautiful Art, user 108128, RRL, Key Flex Aug 2 at 14:29
Please clarify your specific problem or add additional details to highlight exactly what you need. As it's currently written, itâÂÂs hard to tell exactly what you're asking. See the How to Ask page for help clarifying this question. If this question can be reworded to fit the rules in the help center, please edit the question.
To me it appears to be something like $O(fraclognn)$.
â old
Aug 2 at 12:01
3
If we let $n$ tend to infinity, the resulting value can't really depend on $n$.
â Arthur
Aug 2 at 12:01
2
The upper limit should be a natural number. Do you mean perhaps $lfloor log nrfloor$ ?
â Peter
Aug 2 at 12:01
1
I am fine with Order notation also.
â lovw
Aug 2 at 12:01
3
I think the OP is not looking for the limit per se, but the asymptotic growth of $S(n)$, i.e. with respect to $n$.
â Eff
Aug 2 at 12:05
 |Â
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To me it appears to be something like $O(fraclognn)$.
â old
Aug 2 at 12:01
3
If we let $n$ tend to infinity, the resulting value can't really depend on $n$.
â Arthur
Aug 2 at 12:01
2
The upper limit should be a natural number. Do you mean perhaps $lfloor log nrfloor$ ?
â Peter
Aug 2 at 12:01
1
I am fine with Order notation also.
â lovw
Aug 2 at 12:01
3
I think the OP is not looking for the limit per se, but the asymptotic growth of $S(n)$, i.e. with respect to $n$.
â Eff
Aug 2 at 12:05
To me it appears to be something like $O(fraclognn)$.
â old
Aug 2 at 12:01
To me it appears to be something like $O(fraclognn)$.
â old
Aug 2 at 12:01
3
3
If we let $n$ tend to infinity, the resulting value can't really depend on $n$.
â Arthur
Aug 2 at 12:01
If we let $n$ tend to infinity, the resulting value can't really depend on $n$.
â Arthur
Aug 2 at 12:01
2
2
The upper limit should be a natural number. Do you mean perhaps $lfloor log nrfloor$ ?
â Peter
Aug 2 at 12:01
The upper limit should be a natural number. Do you mean perhaps $lfloor log nrfloor$ ?
â Peter
Aug 2 at 12:01
1
1
I am fine with Order notation also.
â lovw
Aug 2 at 12:01
I am fine with Order notation also.
â lovw
Aug 2 at 12:01
3
3
I think the OP is not looking for the limit per se, but the asymptotic growth of $S(n)$, i.e. with respect to $n$.
â Eff
Aug 2 at 12:05
I think the OP is not looking for the limit per se, but the asymptotic growth of $S(n)$, i.e. with respect to $n$.
â Eff
Aug 2 at 12:05
 |Â
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3 Answers
3
active
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votes
up vote
4
down vote
If you mean$$sum_i=1^lfloor log nrfloorfrac i2^i,$$then what you haee is a subsequence of the sequence$$left(sum_i=1^nfrac i2^iright)_ninmathbb N,$$whose sum is $2$. Therefore, the limit of your sequence is $2$, too.
4
I think the OP is not looking for the limit per se, but the asymptotic growth of $S(n)$, i.e. with respect to $n$. However, I might be mistaken.
â Eff
Aug 2 at 12:06
2
@Elf Let us wait and see what the OP has to say about that.
â José Carlos Santos
Aug 2 at 12:07
add a comment |Â
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1
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Consider for $|x|<1$
$$frac ddxsum_i=0^infty x^i=sum_i=1^infty ix^(i-1)=frac ddxleft(frac11-xright)=frac1(1-x)^2$$
then
$$sum_i=1^infty ix^i=xsum_i=1^infty ix^(i-1)=fracx(1-x)^2$$
therefore
$$sum_i=1^inftyfrac i2^i=2$$
and then
$$lim_nto infty sum_i=1^lfloor log nrfloorfrac i2^i=2$$
For the asymptotic grow we have that
$$frac ddxsum_i=0^N x^i=sum_i=1^N ix^(i-1)=frac ddxleft(frac1-x^N+11-xright)=frac1(1-x)^2-fracx^N(1-x)^2(1+N-Nx)$$
and then
$$sum_i=1^N ix^i=xsum_i=1^N ix^(i-1)=fracx(1-x)^2-fracx(1-x)^2x^N(1+N-Nx)$$
and then for $x=frac12$
$$sum_i=1^Nfrac i2^i=2-2frac12^Nleft(1+frac N 2right)=2-frac22^N-fracN2^N=2+Oleft(fracN2^Nright)$$
@lovw If the value of the limit is not what you want, then you should be clearer in the question
â Calvin Khor
Aug 2 at 12:07
add a comment |Â
up vote
1
down vote
Hint. Show by induction that for $Ngeq 1$,
$$sum_i=1^N fraci2^i=2-fracN+22^N.$$
Then the asymptotic growth is
$$S(n)=sum_i=1^lfloor log nrfloor fraci2^i=2-fraclfloor log nrfloor+22^lfloor log nrfloorsim 2-fraclog nn^log 2.$$
add a comment |Â
3 Answers
3
active
oldest
votes
3 Answers
3
active
oldest
votes
active
oldest
votes
active
oldest
votes
up vote
4
down vote
If you mean$$sum_i=1^lfloor log nrfloorfrac i2^i,$$then what you haee is a subsequence of the sequence$$left(sum_i=1^nfrac i2^iright)_ninmathbb N,$$whose sum is $2$. Therefore, the limit of your sequence is $2$, too.
4
I think the OP is not looking for the limit per se, but the asymptotic growth of $S(n)$, i.e. with respect to $n$. However, I might be mistaken.
â Eff
Aug 2 at 12:06
2
@Elf Let us wait and see what the OP has to say about that.
â José Carlos Santos
Aug 2 at 12:07
add a comment |Â
up vote
4
down vote
If you mean$$sum_i=1^lfloor log nrfloorfrac i2^i,$$then what you haee is a subsequence of the sequence$$left(sum_i=1^nfrac i2^iright)_ninmathbb N,$$whose sum is $2$. Therefore, the limit of your sequence is $2$, too.
4
I think the OP is not looking for the limit per se, but the asymptotic growth of $S(n)$, i.e. with respect to $n$. However, I might be mistaken.
â Eff
Aug 2 at 12:06
2
@Elf Let us wait and see what the OP has to say about that.
â José Carlos Santos
Aug 2 at 12:07
add a comment |Â
up vote
4
down vote
up vote
4
down vote
If you mean$$sum_i=1^lfloor log nrfloorfrac i2^i,$$then what you haee is a subsequence of the sequence$$left(sum_i=1^nfrac i2^iright)_ninmathbb N,$$whose sum is $2$. Therefore, the limit of your sequence is $2$, too.
If you mean$$sum_i=1^lfloor log nrfloorfrac i2^i,$$then what you haee is a subsequence of the sequence$$left(sum_i=1^nfrac i2^iright)_ninmathbb N,$$whose sum is $2$. Therefore, the limit of your sequence is $2$, too.
answered Aug 2 at 12:03
José Carlos Santos
112k1696172
112k1696172
4
I think the OP is not looking for the limit per se, but the asymptotic growth of $S(n)$, i.e. with respect to $n$. However, I might be mistaken.
â Eff
Aug 2 at 12:06
2
@Elf Let us wait and see what the OP has to say about that.
â José Carlos Santos
Aug 2 at 12:07
add a comment |Â
4
I think the OP is not looking for the limit per se, but the asymptotic growth of $S(n)$, i.e. with respect to $n$. However, I might be mistaken.
â Eff
Aug 2 at 12:06
2
@Elf Let us wait and see what the OP has to say about that.
â José Carlos Santos
Aug 2 at 12:07
4
4
I think the OP is not looking for the limit per se, but the asymptotic growth of $S(n)$, i.e. with respect to $n$. However, I might be mistaken.
â Eff
Aug 2 at 12:06
I think the OP is not looking for the limit per se, but the asymptotic growth of $S(n)$, i.e. with respect to $n$. However, I might be mistaken.
â Eff
Aug 2 at 12:06
2
2
@Elf Let us wait and see what the OP has to say about that.
â José Carlos Santos
Aug 2 at 12:07
@Elf Let us wait and see what the OP has to say about that.
â José Carlos Santos
Aug 2 at 12:07
add a comment |Â
up vote
1
down vote
Consider for $|x|<1$
$$frac ddxsum_i=0^infty x^i=sum_i=1^infty ix^(i-1)=frac ddxleft(frac11-xright)=frac1(1-x)^2$$
then
$$sum_i=1^infty ix^i=xsum_i=1^infty ix^(i-1)=fracx(1-x)^2$$
therefore
$$sum_i=1^inftyfrac i2^i=2$$
and then
$$lim_nto infty sum_i=1^lfloor log nrfloorfrac i2^i=2$$
For the asymptotic grow we have that
$$frac ddxsum_i=0^N x^i=sum_i=1^N ix^(i-1)=frac ddxleft(frac1-x^N+11-xright)=frac1(1-x)^2-fracx^N(1-x)^2(1+N-Nx)$$
and then
$$sum_i=1^N ix^i=xsum_i=1^N ix^(i-1)=fracx(1-x)^2-fracx(1-x)^2x^N(1+N-Nx)$$
and then for $x=frac12$
$$sum_i=1^Nfrac i2^i=2-2frac12^Nleft(1+frac N 2right)=2-frac22^N-fracN2^N=2+Oleft(fracN2^Nright)$$
@lovw If the value of the limit is not what you want, then you should be clearer in the question
â Calvin Khor
Aug 2 at 12:07
add a comment |Â
up vote
1
down vote
Consider for $|x|<1$
$$frac ddxsum_i=0^infty x^i=sum_i=1^infty ix^(i-1)=frac ddxleft(frac11-xright)=frac1(1-x)^2$$
then
$$sum_i=1^infty ix^i=xsum_i=1^infty ix^(i-1)=fracx(1-x)^2$$
therefore
$$sum_i=1^inftyfrac i2^i=2$$
and then
$$lim_nto infty sum_i=1^lfloor log nrfloorfrac i2^i=2$$
For the asymptotic grow we have that
$$frac ddxsum_i=0^N x^i=sum_i=1^N ix^(i-1)=frac ddxleft(frac1-x^N+11-xright)=frac1(1-x)^2-fracx^N(1-x)^2(1+N-Nx)$$
and then
$$sum_i=1^N ix^i=xsum_i=1^N ix^(i-1)=fracx(1-x)^2-fracx(1-x)^2x^N(1+N-Nx)$$
and then for $x=frac12$
$$sum_i=1^Nfrac i2^i=2-2frac12^Nleft(1+frac N 2right)=2-frac22^N-fracN2^N=2+Oleft(fracN2^Nright)$$
@lovw If the value of the limit is not what you want, then you should be clearer in the question
â Calvin Khor
Aug 2 at 12:07
add a comment |Â
up vote
1
down vote
up vote
1
down vote
Consider for $|x|<1$
$$frac ddxsum_i=0^infty x^i=sum_i=1^infty ix^(i-1)=frac ddxleft(frac11-xright)=frac1(1-x)^2$$
then
$$sum_i=1^infty ix^i=xsum_i=1^infty ix^(i-1)=fracx(1-x)^2$$
therefore
$$sum_i=1^inftyfrac i2^i=2$$
and then
$$lim_nto infty sum_i=1^lfloor log nrfloorfrac i2^i=2$$
For the asymptotic grow we have that
$$frac ddxsum_i=0^N x^i=sum_i=1^N ix^(i-1)=frac ddxleft(frac1-x^N+11-xright)=frac1(1-x)^2-fracx^N(1-x)^2(1+N-Nx)$$
and then
$$sum_i=1^N ix^i=xsum_i=1^N ix^(i-1)=fracx(1-x)^2-fracx(1-x)^2x^N(1+N-Nx)$$
and then for $x=frac12$
$$sum_i=1^Nfrac i2^i=2-2frac12^Nleft(1+frac N 2right)=2-frac22^N-fracN2^N=2+Oleft(fracN2^Nright)$$
Consider for $|x|<1$
$$frac ddxsum_i=0^infty x^i=sum_i=1^infty ix^(i-1)=frac ddxleft(frac11-xright)=frac1(1-x)^2$$
then
$$sum_i=1^infty ix^i=xsum_i=1^infty ix^(i-1)=fracx(1-x)^2$$
therefore
$$sum_i=1^inftyfrac i2^i=2$$
and then
$$lim_nto infty sum_i=1^lfloor log nrfloorfrac i2^i=2$$
For the asymptotic grow we have that
$$frac ddxsum_i=0^N x^i=sum_i=1^N ix^(i-1)=frac ddxleft(frac1-x^N+11-xright)=frac1(1-x)^2-fracx^N(1-x)^2(1+N-Nx)$$
and then
$$sum_i=1^N ix^i=xsum_i=1^N ix^(i-1)=fracx(1-x)^2-fracx(1-x)^2x^N(1+N-Nx)$$
and then for $x=frac12$
$$sum_i=1^Nfrac i2^i=2-2frac12^Nleft(1+frac N 2right)=2-frac22^N-fracN2^N=2+Oleft(fracN2^Nright)$$
edited Aug 2 at 13:19
answered Aug 2 at 12:01
gimusi
63.8k73480
63.8k73480
@lovw If the value of the limit is not what you want, then you should be clearer in the question
â Calvin Khor
Aug 2 at 12:07
add a comment |Â
@lovw If the value of the limit is not what you want, then you should be clearer in the question
â Calvin Khor
Aug 2 at 12:07
@lovw If the value of the limit is not what you want, then you should be clearer in the question
â Calvin Khor
Aug 2 at 12:07
@lovw If the value of the limit is not what you want, then you should be clearer in the question
â Calvin Khor
Aug 2 at 12:07
add a comment |Â
up vote
1
down vote
Hint. Show by induction that for $Ngeq 1$,
$$sum_i=1^N fraci2^i=2-fracN+22^N.$$
Then the asymptotic growth is
$$S(n)=sum_i=1^lfloor log nrfloor fraci2^i=2-fraclfloor log nrfloor+22^lfloor log nrfloorsim 2-fraclog nn^log 2.$$
add a comment |Â
up vote
1
down vote
Hint. Show by induction that for $Ngeq 1$,
$$sum_i=1^N fraci2^i=2-fracN+22^N.$$
Then the asymptotic growth is
$$S(n)=sum_i=1^lfloor log nrfloor fraci2^i=2-fraclfloor log nrfloor+22^lfloor log nrfloorsim 2-fraclog nn^log 2.$$
add a comment |Â
up vote
1
down vote
up vote
1
down vote
Hint. Show by induction that for $Ngeq 1$,
$$sum_i=1^N fraci2^i=2-fracN+22^N.$$
Then the asymptotic growth is
$$S(n)=sum_i=1^lfloor log nrfloor fraci2^i=2-fraclfloor log nrfloor+22^lfloor log nrfloorsim 2-fraclog nn^log 2.$$
Hint. Show by induction that for $Ngeq 1$,
$$sum_i=1^N fraci2^i=2-fracN+22^N.$$
Then the asymptotic growth is
$$S(n)=sum_i=1^lfloor log nrfloor fraci2^i=2-fraclfloor log nrfloor+22^lfloor log nrfloorsim 2-fraclog nn^log 2.$$
edited Aug 2 at 14:39
answered Aug 2 at 12:35
Robert Z
83.5k954122
83.5k954122
add a comment |Â
add a comment |Â
To me it appears to be something like $O(fraclognn)$.
â old
Aug 2 at 12:01
3
If we let $n$ tend to infinity, the resulting value can't really depend on $n$.
â Arthur
Aug 2 at 12:01
2
The upper limit should be a natural number. Do you mean perhaps $lfloor log nrfloor$ ?
â Peter
Aug 2 at 12:01
1
I am fine with Order notation also.
â lovw
Aug 2 at 12:01
3
I think the OP is not looking for the limit per se, but the asymptotic growth of $S(n)$, i.e. with respect to $n$.
â Eff
Aug 2 at 12:05