What does $1+frac{1}{8}+frac{1}{27}+frac{1}{64}+frac{1}{125}+(frac{1}{n})^3$ equal to?
$begingroup$
I'm curious of what does this sum:
$1+frac{1}{8}+frac{1}{27}+frac{1}{64}+frac{1}{125}+frac{1}{216}+...+(frac{1}{n})^3$
or the Riemann zeta function: $zeta({3})$ approach. I watched a few 3Blue1Brown videos on YouTube, but it doesn't have any videos about what does that sum above approach. I don't know how to prove it, and I tried, but suddenly all of my tries lead to no results. Is this problem unsolved? If not, how would you prove it?
riemann-zeta
$endgroup$
add a comment |
$begingroup$
I'm curious of what does this sum:
$1+frac{1}{8}+frac{1}{27}+frac{1}{64}+frac{1}{125}+frac{1}{216}+...+(frac{1}{n})^3$
or the Riemann zeta function: $zeta({3})$ approach. I watched a few 3Blue1Brown videos on YouTube, but it doesn't have any videos about what does that sum above approach. I don't know how to prove it, and I tried, but suddenly all of my tries lead to no results. Is this problem unsolved? If not, how would you prove it?
riemann-zeta
$endgroup$
$begingroup$
It approaches, by definition, $zeta(3)ldots$ There's nothing to prove, except perhaps that the sum converges (obvious by integral test)
$endgroup$
– Brevan Ellefsen
Nov 4 '18 at 22:30
add a comment |
$begingroup$
I'm curious of what does this sum:
$1+frac{1}{8}+frac{1}{27}+frac{1}{64}+frac{1}{125}+frac{1}{216}+...+(frac{1}{n})^3$
or the Riemann zeta function: $zeta({3})$ approach. I watched a few 3Blue1Brown videos on YouTube, but it doesn't have any videos about what does that sum above approach. I don't know how to prove it, and I tried, but suddenly all of my tries lead to no results. Is this problem unsolved? If not, how would you prove it?
riemann-zeta
$endgroup$
I'm curious of what does this sum:
$1+frac{1}{8}+frac{1}{27}+frac{1}{64}+frac{1}{125}+frac{1}{216}+...+(frac{1}{n})^3$
or the Riemann zeta function: $zeta({3})$ approach. I watched a few 3Blue1Brown videos on YouTube, but it doesn't have any videos about what does that sum above approach. I don't know how to prove it, and I tried, but suddenly all of my tries lead to no results. Is this problem unsolved? If not, how would you prove it?
riemann-zeta
riemann-zeta
edited Jan 27 at 23:15
I-85a
asked Nov 4 '18 at 22:27
I-85aI-85a
83
83
$begingroup$
It approaches, by definition, $zeta(3)ldots$ There's nothing to prove, except perhaps that the sum converges (obvious by integral test)
$endgroup$
– Brevan Ellefsen
Nov 4 '18 at 22:30
add a comment |
$begingroup$
It approaches, by definition, $zeta(3)ldots$ There's nothing to prove, except perhaps that the sum converges (obvious by integral test)
$endgroup$
– Brevan Ellefsen
Nov 4 '18 at 22:30
$begingroup$
It approaches, by definition, $zeta(3)ldots$ There's nothing to prove, except perhaps that the sum converges (obvious by integral test)
$endgroup$
– Brevan Ellefsen
Nov 4 '18 at 22:30
$begingroup$
It approaches, by definition, $zeta(3)ldots$ There's nothing to prove, except perhaps that the sum converges (obvious by integral test)
$endgroup$
– Brevan Ellefsen
Nov 4 '18 at 22:30
add a comment |
2 Answers
2
active
oldest
votes
$begingroup$
$zeta(3)$ is a constant known as Apery's constant. Its value is approximately $1.2021ldots$ but as far as I know an exact value isn't known.
Some further reading:
https://en.wikipedia.org/wiki/Ap%C3%A9ry%27s_constant
http://mathworld.wolfram.com/AperysConstant.html
$endgroup$
$begingroup$
I don't know what it is you think "isn't quite known". The "exact value" is $zeta(3)$. Apéry proved it is irrational.
$endgroup$
– Robert Israel
Nov 4 '18 at 22:49
$begingroup$
"The exact value of $zeta(3)$ is $zeta(3)$" isn't exactly an enlightening answer. I mean something in terms of - at least more known - constants, i.e. how we know $zeta(2) = pi^2/6$. A "closed form" value, perhaps? Whatever the term is.
$endgroup$
– Eevee Trainer
Nov 4 '18 at 22:51
$begingroup$
It is not likely that $zeta(3)$ has a closed form in terms of "better-known" constants, though of course this has not been proven. We don't even have a proof that it is transcendental.
$endgroup$
– Robert Israel
Nov 4 '18 at 23:00
$begingroup$
Well, there is $zeta(3) = -Psi''(1)/2$, but I don't know if you'd count that as "better-known".
$endgroup$
– Robert Israel
Nov 5 '18 at 2:26
$begingroup$
"The exact value of ζ(3) is ζ(3)" isn't exactly an enlightening answer." Why not? Why would not you say "the exact value of $pi$ isn't quite known"?
$endgroup$
– user
Nov 5 '18 at 22:07
add a comment |
$begingroup$
Well, $sum_{ngeq 1}frac{1}{n^3}$ is a number, precisely the value of the Riemann $zeta$ function at $s=3$. Since $xleq text{arctanh}(x)$ is a pretty tight approximation for $xto 0^+$,
$$zeta(3) = 1+sum_{ngeq 2}frac{1}{n^3} leq 1+frac{1}{2}sum_{ngeq 2}logleft(frac{n^3+1}{n^3-1}right)$$
but $prod_{ngeq 2}frac{n^3+1}{n^3-1}=prod_{ngeq 2}frac{n+1}{n-1}cdotfrac{n^2-n+1}{n^2+n+1}$ is a telescopic product convergent to $frac{3}{2}$, hence
$$ zeta(3) approx 1+frac{log 3-log 2}{2}.$$
By creative telescoping we also have the acceleration formula
$$ zeta(3)=sum_{ngeq 1}frac{1}{n^3}=frac{5}{2}sum_{ngeq 1}frac{(-1)^{n+1}}{n^3binom{2n}{n}}$$
(see my notes for a proof) which allowed Apery to prove that $zeta(3)notinmathbb{Q}$.
The irrationality of $zeta(5),zeta(7),zeta(9),ldots$ still is an open problem, like the conjecture $zeta(3)inpi^3mathbb{Q}$, which looks numerically very unlikely. On the other hand $sum_{ngeq 0}frac{(-1)^n}{(2n+1)^3}inpi^3mathbb{Q}$, as shown here.
$endgroup$
add a comment |
Your Answer
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2 Answers
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2 Answers
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$begingroup$
$zeta(3)$ is a constant known as Apery's constant. Its value is approximately $1.2021ldots$ but as far as I know an exact value isn't known.
Some further reading:
https://en.wikipedia.org/wiki/Ap%C3%A9ry%27s_constant
http://mathworld.wolfram.com/AperysConstant.html
$endgroup$
$begingroup$
I don't know what it is you think "isn't quite known". The "exact value" is $zeta(3)$. Apéry proved it is irrational.
$endgroup$
– Robert Israel
Nov 4 '18 at 22:49
$begingroup$
"The exact value of $zeta(3)$ is $zeta(3)$" isn't exactly an enlightening answer. I mean something in terms of - at least more known - constants, i.e. how we know $zeta(2) = pi^2/6$. A "closed form" value, perhaps? Whatever the term is.
$endgroup$
– Eevee Trainer
Nov 4 '18 at 22:51
$begingroup$
It is not likely that $zeta(3)$ has a closed form in terms of "better-known" constants, though of course this has not been proven. We don't even have a proof that it is transcendental.
$endgroup$
– Robert Israel
Nov 4 '18 at 23:00
$begingroup$
Well, there is $zeta(3) = -Psi''(1)/2$, but I don't know if you'd count that as "better-known".
$endgroup$
– Robert Israel
Nov 5 '18 at 2:26
$begingroup$
"The exact value of ζ(3) is ζ(3)" isn't exactly an enlightening answer." Why not? Why would not you say "the exact value of $pi$ isn't quite known"?
$endgroup$
– user
Nov 5 '18 at 22:07
add a comment |
$begingroup$
$zeta(3)$ is a constant known as Apery's constant. Its value is approximately $1.2021ldots$ but as far as I know an exact value isn't known.
Some further reading:
https://en.wikipedia.org/wiki/Ap%C3%A9ry%27s_constant
http://mathworld.wolfram.com/AperysConstant.html
$endgroup$
$begingroup$
I don't know what it is you think "isn't quite known". The "exact value" is $zeta(3)$. Apéry proved it is irrational.
$endgroup$
– Robert Israel
Nov 4 '18 at 22:49
$begingroup$
"The exact value of $zeta(3)$ is $zeta(3)$" isn't exactly an enlightening answer. I mean something in terms of - at least more known - constants, i.e. how we know $zeta(2) = pi^2/6$. A "closed form" value, perhaps? Whatever the term is.
$endgroup$
– Eevee Trainer
Nov 4 '18 at 22:51
$begingroup$
It is not likely that $zeta(3)$ has a closed form in terms of "better-known" constants, though of course this has not been proven. We don't even have a proof that it is transcendental.
$endgroup$
– Robert Israel
Nov 4 '18 at 23:00
$begingroup$
Well, there is $zeta(3) = -Psi''(1)/2$, but I don't know if you'd count that as "better-known".
$endgroup$
– Robert Israel
Nov 5 '18 at 2:26
$begingroup$
"The exact value of ζ(3) is ζ(3)" isn't exactly an enlightening answer." Why not? Why would not you say "the exact value of $pi$ isn't quite known"?
$endgroup$
– user
Nov 5 '18 at 22:07
add a comment |
$begingroup$
$zeta(3)$ is a constant known as Apery's constant. Its value is approximately $1.2021ldots$ but as far as I know an exact value isn't known.
Some further reading:
https://en.wikipedia.org/wiki/Ap%C3%A9ry%27s_constant
http://mathworld.wolfram.com/AperysConstant.html
$endgroup$
$zeta(3)$ is a constant known as Apery's constant. Its value is approximately $1.2021ldots$ but as far as I know an exact value isn't known.
Some further reading:
https://en.wikipedia.org/wiki/Ap%C3%A9ry%27s_constant
http://mathworld.wolfram.com/AperysConstant.html
edited Dec 13 '18 at 9:02
answered Nov 4 '18 at 22:29
Eevee TrainerEevee Trainer
6,80311237
6,80311237
$begingroup$
I don't know what it is you think "isn't quite known". The "exact value" is $zeta(3)$. Apéry proved it is irrational.
$endgroup$
– Robert Israel
Nov 4 '18 at 22:49
$begingroup$
"The exact value of $zeta(3)$ is $zeta(3)$" isn't exactly an enlightening answer. I mean something in terms of - at least more known - constants, i.e. how we know $zeta(2) = pi^2/6$. A "closed form" value, perhaps? Whatever the term is.
$endgroup$
– Eevee Trainer
Nov 4 '18 at 22:51
$begingroup$
It is not likely that $zeta(3)$ has a closed form in terms of "better-known" constants, though of course this has not been proven. We don't even have a proof that it is transcendental.
$endgroup$
– Robert Israel
Nov 4 '18 at 23:00
$begingroup$
Well, there is $zeta(3) = -Psi''(1)/2$, but I don't know if you'd count that as "better-known".
$endgroup$
– Robert Israel
Nov 5 '18 at 2:26
$begingroup$
"The exact value of ζ(3) is ζ(3)" isn't exactly an enlightening answer." Why not? Why would not you say "the exact value of $pi$ isn't quite known"?
$endgroup$
– user
Nov 5 '18 at 22:07
add a comment |
$begingroup$
I don't know what it is you think "isn't quite known". The "exact value" is $zeta(3)$. Apéry proved it is irrational.
$endgroup$
– Robert Israel
Nov 4 '18 at 22:49
$begingroup$
"The exact value of $zeta(3)$ is $zeta(3)$" isn't exactly an enlightening answer. I mean something in terms of - at least more known - constants, i.e. how we know $zeta(2) = pi^2/6$. A "closed form" value, perhaps? Whatever the term is.
$endgroup$
– Eevee Trainer
Nov 4 '18 at 22:51
$begingroup$
It is not likely that $zeta(3)$ has a closed form in terms of "better-known" constants, though of course this has not been proven. We don't even have a proof that it is transcendental.
$endgroup$
– Robert Israel
Nov 4 '18 at 23:00
$begingroup$
Well, there is $zeta(3) = -Psi''(1)/2$, but I don't know if you'd count that as "better-known".
$endgroup$
– Robert Israel
Nov 5 '18 at 2:26
$begingroup$
"The exact value of ζ(3) is ζ(3)" isn't exactly an enlightening answer." Why not? Why would not you say "the exact value of $pi$ isn't quite known"?
$endgroup$
– user
Nov 5 '18 at 22:07
$begingroup$
I don't know what it is you think "isn't quite known". The "exact value" is $zeta(3)$. Apéry proved it is irrational.
$endgroup$
– Robert Israel
Nov 4 '18 at 22:49
$begingroup$
I don't know what it is you think "isn't quite known". The "exact value" is $zeta(3)$. Apéry proved it is irrational.
$endgroup$
– Robert Israel
Nov 4 '18 at 22:49
$begingroup$
"The exact value of $zeta(3)$ is $zeta(3)$" isn't exactly an enlightening answer. I mean something in terms of - at least more known - constants, i.e. how we know $zeta(2) = pi^2/6$. A "closed form" value, perhaps? Whatever the term is.
$endgroup$
– Eevee Trainer
Nov 4 '18 at 22:51
$begingroup$
"The exact value of $zeta(3)$ is $zeta(3)$" isn't exactly an enlightening answer. I mean something in terms of - at least more known - constants, i.e. how we know $zeta(2) = pi^2/6$. A "closed form" value, perhaps? Whatever the term is.
$endgroup$
– Eevee Trainer
Nov 4 '18 at 22:51
$begingroup$
It is not likely that $zeta(3)$ has a closed form in terms of "better-known" constants, though of course this has not been proven. We don't even have a proof that it is transcendental.
$endgroup$
– Robert Israel
Nov 4 '18 at 23:00
$begingroup$
It is not likely that $zeta(3)$ has a closed form in terms of "better-known" constants, though of course this has not been proven. We don't even have a proof that it is transcendental.
$endgroup$
– Robert Israel
Nov 4 '18 at 23:00
$begingroup$
Well, there is $zeta(3) = -Psi''(1)/2$, but I don't know if you'd count that as "better-known".
$endgroup$
– Robert Israel
Nov 5 '18 at 2:26
$begingroup$
Well, there is $zeta(3) = -Psi''(1)/2$, but I don't know if you'd count that as "better-known".
$endgroup$
– Robert Israel
Nov 5 '18 at 2:26
$begingroup$
"The exact value of ζ(3) is ζ(3)" isn't exactly an enlightening answer." Why not? Why would not you say "the exact value of $pi$ isn't quite known"?
$endgroup$
– user
Nov 5 '18 at 22:07
$begingroup$
"The exact value of ζ(3) is ζ(3)" isn't exactly an enlightening answer." Why not? Why would not you say "the exact value of $pi$ isn't quite known"?
$endgroup$
– user
Nov 5 '18 at 22:07
add a comment |
$begingroup$
Well, $sum_{ngeq 1}frac{1}{n^3}$ is a number, precisely the value of the Riemann $zeta$ function at $s=3$. Since $xleq text{arctanh}(x)$ is a pretty tight approximation for $xto 0^+$,
$$zeta(3) = 1+sum_{ngeq 2}frac{1}{n^3} leq 1+frac{1}{2}sum_{ngeq 2}logleft(frac{n^3+1}{n^3-1}right)$$
but $prod_{ngeq 2}frac{n^3+1}{n^3-1}=prod_{ngeq 2}frac{n+1}{n-1}cdotfrac{n^2-n+1}{n^2+n+1}$ is a telescopic product convergent to $frac{3}{2}$, hence
$$ zeta(3) approx 1+frac{log 3-log 2}{2}.$$
By creative telescoping we also have the acceleration formula
$$ zeta(3)=sum_{ngeq 1}frac{1}{n^3}=frac{5}{2}sum_{ngeq 1}frac{(-1)^{n+1}}{n^3binom{2n}{n}}$$
(see my notes for a proof) which allowed Apery to prove that $zeta(3)notinmathbb{Q}$.
The irrationality of $zeta(5),zeta(7),zeta(9),ldots$ still is an open problem, like the conjecture $zeta(3)inpi^3mathbb{Q}$, which looks numerically very unlikely. On the other hand $sum_{ngeq 0}frac{(-1)^n}{(2n+1)^3}inpi^3mathbb{Q}$, as shown here.
$endgroup$
add a comment |
$begingroup$
Well, $sum_{ngeq 1}frac{1}{n^3}$ is a number, precisely the value of the Riemann $zeta$ function at $s=3$. Since $xleq text{arctanh}(x)$ is a pretty tight approximation for $xto 0^+$,
$$zeta(3) = 1+sum_{ngeq 2}frac{1}{n^3} leq 1+frac{1}{2}sum_{ngeq 2}logleft(frac{n^3+1}{n^3-1}right)$$
but $prod_{ngeq 2}frac{n^3+1}{n^3-1}=prod_{ngeq 2}frac{n+1}{n-1}cdotfrac{n^2-n+1}{n^2+n+1}$ is a telescopic product convergent to $frac{3}{2}$, hence
$$ zeta(3) approx 1+frac{log 3-log 2}{2}.$$
By creative telescoping we also have the acceleration formula
$$ zeta(3)=sum_{ngeq 1}frac{1}{n^3}=frac{5}{2}sum_{ngeq 1}frac{(-1)^{n+1}}{n^3binom{2n}{n}}$$
(see my notes for a proof) which allowed Apery to prove that $zeta(3)notinmathbb{Q}$.
The irrationality of $zeta(5),zeta(7),zeta(9),ldots$ still is an open problem, like the conjecture $zeta(3)inpi^3mathbb{Q}$, which looks numerically very unlikely. On the other hand $sum_{ngeq 0}frac{(-1)^n}{(2n+1)^3}inpi^3mathbb{Q}$, as shown here.
$endgroup$
add a comment |
$begingroup$
Well, $sum_{ngeq 1}frac{1}{n^3}$ is a number, precisely the value of the Riemann $zeta$ function at $s=3$. Since $xleq text{arctanh}(x)$ is a pretty tight approximation for $xto 0^+$,
$$zeta(3) = 1+sum_{ngeq 2}frac{1}{n^3} leq 1+frac{1}{2}sum_{ngeq 2}logleft(frac{n^3+1}{n^3-1}right)$$
but $prod_{ngeq 2}frac{n^3+1}{n^3-1}=prod_{ngeq 2}frac{n+1}{n-1}cdotfrac{n^2-n+1}{n^2+n+1}$ is a telescopic product convergent to $frac{3}{2}$, hence
$$ zeta(3) approx 1+frac{log 3-log 2}{2}.$$
By creative telescoping we also have the acceleration formula
$$ zeta(3)=sum_{ngeq 1}frac{1}{n^3}=frac{5}{2}sum_{ngeq 1}frac{(-1)^{n+1}}{n^3binom{2n}{n}}$$
(see my notes for a proof) which allowed Apery to prove that $zeta(3)notinmathbb{Q}$.
The irrationality of $zeta(5),zeta(7),zeta(9),ldots$ still is an open problem, like the conjecture $zeta(3)inpi^3mathbb{Q}$, which looks numerically very unlikely. On the other hand $sum_{ngeq 0}frac{(-1)^n}{(2n+1)^3}inpi^3mathbb{Q}$, as shown here.
$endgroup$
Well, $sum_{ngeq 1}frac{1}{n^3}$ is a number, precisely the value of the Riemann $zeta$ function at $s=3$. Since $xleq text{arctanh}(x)$ is a pretty tight approximation for $xto 0^+$,
$$zeta(3) = 1+sum_{ngeq 2}frac{1}{n^3} leq 1+frac{1}{2}sum_{ngeq 2}logleft(frac{n^3+1}{n^3-1}right)$$
but $prod_{ngeq 2}frac{n^3+1}{n^3-1}=prod_{ngeq 2}frac{n+1}{n-1}cdotfrac{n^2-n+1}{n^2+n+1}$ is a telescopic product convergent to $frac{3}{2}$, hence
$$ zeta(3) approx 1+frac{log 3-log 2}{2}.$$
By creative telescoping we also have the acceleration formula
$$ zeta(3)=sum_{ngeq 1}frac{1}{n^3}=frac{5}{2}sum_{ngeq 1}frac{(-1)^{n+1}}{n^3binom{2n}{n}}$$
(see my notes for a proof) which allowed Apery to prove that $zeta(3)notinmathbb{Q}$.
The irrationality of $zeta(5),zeta(7),zeta(9),ldots$ still is an open problem, like the conjecture $zeta(3)inpi^3mathbb{Q}$, which looks numerically very unlikely. On the other hand $sum_{ngeq 0}frac{(-1)^n}{(2n+1)^3}inpi^3mathbb{Q}$, as shown here.
answered Nov 4 '18 at 23:31
Jack D'AurizioJack D'Aurizio
290k33282664
290k33282664
add a comment |
add a comment |
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$begingroup$
It approaches, by definition, $zeta(3)ldots$ There's nothing to prove, except perhaps that the sum converges (obvious by integral test)
$endgroup$
– Brevan Ellefsen
Nov 4 '18 at 22:30