A sum of Fibonacci numbers
$begingroup$
Let $F_n$ be the Fibonacci numbers. I would like to prove this really messy identity:
$$sum_{n=0}^{infty}(-1)^n(n+1)^2frac{5n^2F_{k}+(F_{k+3}+3F_{k+2})n+2F_{k+2}+F_{k-1}}{{2n+2choose 1}{2n+2 choose 2}{2n choose n}}=F_{k}tag1$$
which simplifies to:
$$sum_{n=0}^{infty}(-1)^nfrac{5n^2F_{k}+(F_{k+3}+3F_{k+2})n+2F_{k+2}+F_{k-1}}{{2n choose n}(4n+2)}=F_{k}$$
How can I evaluate the following sums to show this identity (1) is correct?
$$sum_{n=0}^{infty}(-1)^nfrac{n^2}{{2n choose n}(2n+1)}=A$$
$$sum_{n=0}^{infty}(-1)^nfrac{n}{{2n choose n}(2n+1)}=B$$
$$sum_{n=0}^{infty}(-1)^nfrac{1}{{2n choose n}(2n+1)}=C$$
sequences-and-series fibonacci-numbers
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coffeee is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.
$endgroup$
add a comment |
$begingroup$
Let $F_n$ be the Fibonacci numbers. I would like to prove this really messy identity:
$$sum_{n=0}^{infty}(-1)^n(n+1)^2frac{5n^2F_{k}+(F_{k+3}+3F_{k+2})n+2F_{k+2}+F_{k-1}}{{2n+2choose 1}{2n+2 choose 2}{2n choose n}}=F_{k}tag1$$
which simplifies to:
$$sum_{n=0}^{infty}(-1)^nfrac{5n^2F_{k}+(F_{k+3}+3F_{k+2})n+2F_{k+2}+F_{k-1}}{{2n choose n}(4n+2)}=F_{k}$$
How can I evaluate the following sums to show this identity (1) is correct?
$$sum_{n=0}^{infty}(-1)^nfrac{n^2}{{2n choose n}(2n+1)}=A$$
$$sum_{n=0}^{infty}(-1)^nfrac{n}{{2n choose n}(2n+1)}=B$$
$$sum_{n=0}^{infty}(-1)^nfrac{1}{{2n choose n}(2n+1)}=C$$
sequences-and-series fibonacci-numbers
New contributor
coffeee is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.
$endgroup$
6
$begingroup$
Wow, how did you come across this monstrosity?
$endgroup$
– YiFan
3 hours ago
add a comment |
$begingroup$
Let $F_n$ be the Fibonacci numbers. I would like to prove this really messy identity:
$$sum_{n=0}^{infty}(-1)^n(n+1)^2frac{5n^2F_{k}+(F_{k+3}+3F_{k+2})n+2F_{k+2}+F_{k-1}}{{2n+2choose 1}{2n+2 choose 2}{2n choose n}}=F_{k}tag1$$
which simplifies to:
$$sum_{n=0}^{infty}(-1)^nfrac{5n^2F_{k}+(F_{k+3}+3F_{k+2})n+2F_{k+2}+F_{k-1}}{{2n choose n}(4n+2)}=F_{k}$$
How can I evaluate the following sums to show this identity (1) is correct?
$$sum_{n=0}^{infty}(-1)^nfrac{n^2}{{2n choose n}(2n+1)}=A$$
$$sum_{n=0}^{infty}(-1)^nfrac{n}{{2n choose n}(2n+1)}=B$$
$$sum_{n=0}^{infty}(-1)^nfrac{1}{{2n choose n}(2n+1)}=C$$
sequences-and-series fibonacci-numbers
New contributor
coffeee is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.
$endgroup$
Let $F_n$ be the Fibonacci numbers. I would like to prove this really messy identity:
$$sum_{n=0}^{infty}(-1)^n(n+1)^2frac{5n^2F_{k}+(F_{k+3}+3F_{k+2})n+2F_{k+2}+F_{k-1}}{{2n+2choose 1}{2n+2 choose 2}{2n choose n}}=F_{k}tag1$$
which simplifies to:
$$sum_{n=0}^{infty}(-1)^nfrac{5n^2F_{k}+(F_{k+3}+3F_{k+2})n+2F_{k+2}+F_{k-1}}{{2n choose n}(4n+2)}=F_{k}$$
How can I evaluate the following sums to show this identity (1) is correct?
$$sum_{n=0}^{infty}(-1)^nfrac{n^2}{{2n choose n}(2n+1)}=A$$
$$sum_{n=0}^{infty}(-1)^nfrac{n}{{2n choose n}(2n+1)}=B$$
$$sum_{n=0}^{infty}(-1)^nfrac{1}{{2n choose n}(2n+1)}=C$$
sequences-and-series fibonacci-numbers
sequences-and-series fibonacci-numbers
New contributor
coffeee is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.
New contributor
coffeee is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.
edited 1 hour ago
J. W. Tanner
1,308114
1,308114
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coffeee is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
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asked 3 hours ago
coffeeecoffeee
311
311
New contributor
coffeee is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
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New contributor
coffeee is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.
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Check out our Code of Conduct.
6
$begingroup$
Wow, how did you come across this monstrosity?
$endgroup$
– YiFan
3 hours ago
add a comment |
6
$begingroup$
Wow, how did you come across this monstrosity?
$endgroup$
– YiFan
3 hours ago
6
6
$begingroup$
Wow, how did you come across this monstrosity?
$endgroup$
– YiFan
3 hours ago
$begingroup$
Wow, how did you come across this monstrosity?
$endgroup$
– YiFan
3 hours ago
add a comment |
1 Answer
1
active
oldest
votes
$begingroup$
If we use Wolfram for these sums, with approximate values
begin{align*}
C approx 0.86082, qquad B approx -0.11649, qquad A approx -0.07897
end{align*}
From there, you can collect coefficients of $F_r$ to prove your identity. A more concrete way to derive exact expressions is through generating functions. Define
begin{align*}
g(x) = sum_{n=0}^{infty}frac{(-1)^n}{binom{2n}{n}(2n+1)}x^n
end{align*}
which you can show equals
begin{align*}
g(x) = 4 frac{sinh^{-1}(sqrt{x}/2)}{sqrt{x(x+4)}}
end{align*}
If we define $L = x frac{d}{dx}$ as the operator which first takes the derivative w.r.t $x$, then multiplies by $x$, we get that
begin{align*}
C = (L^0 g)(1), qquad B = (L^1 g)(1), qquad A = (L^2 g)(1)
end{align*}
or
begin{align*}
C = 4 frac{sinh^{-1}(1/2)}{sqrt{5}}, qquad B = frac{2}{5} - frac{12 sinh^{-1}(1/2)}{5sqrt{5}}, qquad A = frac{4}{125}(7sqrt{5}sinh^{-1}(1/2) - 10)
end{align*}
$endgroup$
1
$begingroup$
Awesome answer! How did you come up with the generating functions and the corresponding closed form solution? Can you propose any resource for this?
$endgroup$
– MrYouMath
2 hours ago
1
$begingroup$
@MrYouMath: If you write $binom{2n}{n}=binom{2n}{2n-n}$, then the generating function $g(x)$ is the convolution of coefficients $a_n:=binom{2n}{n}^{-1}$ and $b_n:=(2n+1)^{-1}$, which is equivalent to the product of their individual generating functions.
$endgroup$
– Alex R.
1 hour ago
$begingroup$
Hi @MrYouMath, Alex's method is perhaps the best way to discover the generating function. Sometimes, you can even recognize the coefficients to be similar to existing Taylor expansions. In this case, the $a_n b_n$ collectively has similar form to the coefficients in the Taylor series expansion of $sinh^{-1}(x)$. From there, you can apply formal power series operations to try and and match the coefficients. The standard reference is generatingfunctionology by Wilf. Stanley's Enumerative Combinatorics also has an excellent presentation on generating functions / formal power series.
$endgroup$
– Tom Chen
44 mins ago
add a comment |
Your Answer
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1 Answer
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1 Answer
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active
oldest
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oldest
votes
$begingroup$
If we use Wolfram for these sums, with approximate values
begin{align*}
C approx 0.86082, qquad B approx -0.11649, qquad A approx -0.07897
end{align*}
From there, you can collect coefficients of $F_r$ to prove your identity. A more concrete way to derive exact expressions is through generating functions. Define
begin{align*}
g(x) = sum_{n=0}^{infty}frac{(-1)^n}{binom{2n}{n}(2n+1)}x^n
end{align*}
which you can show equals
begin{align*}
g(x) = 4 frac{sinh^{-1}(sqrt{x}/2)}{sqrt{x(x+4)}}
end{align*}
If we define $L = x frac{d}{dx}$ as the operator which first takes the derivative w.r.t $x$, then multiplies by $x$, we get that
begin{align*}
C = (L^0 g)(1), qquad B = (L^1 g)(1), qquad A = (L^2 g)(1)
end{align*}
or
begin{align*}
C = 4 frac{sinh^{-1}(1/2)}{sqrt{5}}, qquad B = frac{2}{5} - frac{12 sinh^{-1}(1/2)}{5sqrt{5}}, qquad A = frac{4}{125}(7sqrt{5}sinh^{-1}(1/2) - 10)
end{align*}
$endgroup$
1
$begingroup$
Awesome answer! How did you come up with the generating functions and the corresponding closed form solution? Can you propose any resource for this?
$endgroup$
– MrYouMath
2 hours ago
1
$begingroup$
@MrYouMath: If you write $binom{2n}{n}=binom{2n}{2n-n}$, then the generating function $g(x)$ is the convolution of coefficients $a_n:=binom{2n}{n}^{-1}$ and $b_n:=(2n+1)^{-1}$, which is equivalent to the product of their individual generating functions.
$endgroup$
– Alex R.
1 hour ago
$begingroup$
Hi @MrYouMath, Alex's method is perhaps the best way to discover the generating function. Sometimes, you can even recognize the coefficients to be similar to existing Taylor expansions. In this case, the $a_n b_n$ collectively has similar form to the coefficients in the Taylor series expansion of $sinh^{-1}(x)$. From there, you can apply formal power series operations to try and and match the coefficients. The standard reference is generatingfunctionology by Wilf. Stanley's Enumerative Combinatorics also has an excellent presentation on generating functions / formal power series.
$endgroup$
– Tom Chen
44 mins ago
add a comment |
$begingroup$
If we use Wolfram for these sums, with approximate values
begin{align*}
C approx 0.86082, qquad B approx -0.11649, qquad A approx -0.07897
end{align*}
From there, you can collect coefficients of $F_r$ to prove your identity. A more concrete way to derive exact expressions is through generating functions. Define
begin{align*}
g(x) = sum_{n=0}^{infty}frac{(-1)^n}{binom{2n}{n}(2n+1)}x^n
end{align*}
which you can show equals
begin{align*}
g(x) = 4 frac{sinh^{-1}(sqrt{x}/2)}{sqrt{x(x+4)}}
end{align*}
If we define $L = x frac{d}{dx}$ as the operator which first takes the derivative w.r.t $x$, then multiplies by $x$, we get that
begin{align*}
C = (L^0 g)(1), qquad B = (L^1 g)(1), qquad A = (L^2 g)(1)
end{align*}
or
begin{align*}
C = 4 frac{sinh^{-1}(1/2)}{sqrt{5}}, qquad B = frac{2}{5} - frac{12 sinh^{-1}(1/2)}{5sqrt{5}}, qquad A = frac{4}{125}(7sqrt{5}sinh^{-1}(1/2) - 10)
end{align*}
$endgroup$
1
$begingroup$
Awesome answer! How did you come up with the generating functions and the corresponding closed form solution? Can you propose any resource for this?
$endgroup$
– MrYouMath
2 hours ago
1
$begingroup$
@MrYouMath: If you write $binom{2n}{n}=binom{2n}{2n-n}$, then the generating function $g(x)$ is the convolution of coefficients $a_n:=binom{2n}{n}^{-1}$ and $b_n:=(2n+1)^{-1}$, which is equivalent to the product of their individual generating functions.
$endgroup$
– Alex R.
1 hour ago
$begingroup$
Hi @MrYouMath, Alex's method is perhaps the best way to discover the generating function. Sometimes, you can even recognize the coefficients to be similar to existing Taylor expansions. In this case, the $a_n b_n$ collectively has similar form to the coefficients in the Taylor series expansion of $sinh^{-1}(x)$. From there, you can apply formal power series operations to try and and match the coefficients. The standard reference is generatingfunctionology by Wilf. Stanley's Enumerative Combinatorics also has an excellent presentation on generating functions / formal power series.
$endgroup$
– Tom Chen
44 mins ago
add a comment |
$begingroup$
If we use Wolfram for these sums, with approximate values
begin{align*}
C approx 0.86082, qquad B approx -0.11649, qquad A approx -0.07897
end{align*}
From there, you can collect coefficients of $F_r$ to prove your identity. A more concrete way to derive exact expressions is through generating functions. Define
begin{align*}
g(x) = sum_{n=0}^{infty}frac{(-1)^n}{binom{2n}{n}(2n+1)}x^n
end{align*}
which you can show equals
begin{align*}
g(x) = 4 frac{sinh^{-1}(sqrt{x}/2)}{sqrt{x(x+4)}}
end{align*}
If we define $L = x frac{d}{dx}$ as the operator which first takes the derivative w.r.t $x$, then multiplies by $x$, we get that
begin{align*}
C = (L^0 g)(1), qquad B = (L^1 g)(1), qquad A = (L^2 g)(1)
end{align*}
or
begin{align*}
C = 4 frac{sinh^{-1}(1/2)}{sqrt{5}}, qquad B = frac{2}{5} - frac{12 sinh^{-1}(1/2)}{5sqrt{5}}, qquad A = frac{4}{125}(7sqrt{5}sinh^{-1}(1/2) - 10)
end{align*}
$endgroup$
If we use Wolfram for these sums, with approximate values
begin{align*}
C approx 0.86082, qquad B approx -0.11649, qquad A approx -0.07897
end{align*}
From there, you can collect coefficients of $F_r$ to prove your identity. A more concrete way to derive exact expressions is through generating functions. Define
begin{align*}
g(x) = sum_{n=0}^{infty}frac{(-1)^n}{binom{2n}{n}(2n+1)}x^n
end{align*}
which you can show equals
begin{align*}
g(x) = 4 frac{sinh^{-1}(sqrt{x}/2)}{sqrt{x(x+4)}}
end{align*}
If we define $L = x frac{d}{dx}$ as the operator which first takes the derivative w.r.t $x$, then multiplies by $x$, we get that
begin{align*}
C = (L^0 g)(1), qquad B = (L^1 g)(1), qquad A = (L^2 g)(1)
end{align*}
or
begin{align*}
C = 4 frac{sinh^{-1}(1/2)}{sqrt{5}}, qquad B = frac{2}{5} - frac{12 sinh^{-1}(1/2)}{5sqrt{5}}, qquad A = frac{4}{125}(7sqrt{5}sinh^{-1}(1/2) - 10)
end{align*}
edited 1 hour ago
answered 3 hours ago
Tom ChenTom Chen
858412
858412
1
$begingroup$
Awesome answer! How did you come up with the generating functions and the corresponding closed form solution? Can you propose any resource for this?
$endgroup$
– MrYouMath
2 hours ago
1
$begingroup$
@MrYouMath: If you write $binom{2n}{n}=binom{2n}{2n-n}$, then the generating function $g(x)$ is the convolution of coefficients $a_n:=binom{2n}{n}^{-1}$ and $b_n:=(2n+1)^{-1}$, which is equivalent to the product of their individual generating functions.
$endgroup$
– Alex R.
1 hour ago
$begingroup$
Hi @MrYouMath, Alex's method is perhaps the best way to discover the generating function. Sometimes, you can even recognize the coefficients to be similar to existing Taylor expansions. In this case, the $a_n b_n$ collectively has similar form to the coefficients in the Taylor series expansion of $sinh^{-1}(x)$. From there, you can apply formal power series operations to try and and match the coefficients. The standard reference is generatingfunctionology by Wilf. Stanley's Enumerative Combinatorics also has an excellent presentation on generating functions / formal power series.
$endgroup$
– Tom Chen
44 mins ago
add a comment |
1
$begingroup$
Awesome answer! How did you come up with the generating functions and the corresponding closed form solution? Can you propose any resource for this?
$endgroup$
– MrYouMath
2 hours ago
1
$begingroup$
@MrYouMath: If you write $binom{2n}{n}=binom{2n}{2n-n}$, then the generating function $g(x)$ is the convolution of coefficients $a_n:=binom{2n}{n}^{-1}$ and $b_n:=(2n+1)^{-1}$, which is equivalent to the product of their individual generating functions.
$endgroup$
– Alex R.
1 hour ago
$begingroup$
Hi @MrYouMath, Alex's method is perhaps the best way to discover the generating function. Sometimes, you can even recognize the coefficients to be similar to existing Taylor expansions. In this case, the $a_n b_n$ collectively has similar form to the coefficients in the Taylor series expansion of $sinh^{-1}(x)$. From there, you can apply formal power series operations to try and and match the coefficients. The standard reference is generatingfunctionology by Wilf. Stanley's Enumerative Combinatorics also has an excellent presentation on generating functions / formal power series.
$endgroup$
– Tom Chen
44 mins ago
1
1
$begingroup$
Awesome answer! How did you come up with the generating functions and the corresponding closed form solution? Can you propose any resource for this?
$endgroup$
– MrYouMath
2 hours ago
$begingroup$
Awesome answer! How did you come up with the generating functions and the corresponding closed form solution? Can you propose any resource for this?
$endgroup$
– MrYouMath
2 hours ago
1
1
$begingroup$
@MrYouMath: If you write $binom{2n}{n}=binom{2n}{2n-n}$, then the generating function $g(x)$ is the convolution of coefficients $a_n:=binom{2n}{n}^{-1}$ and $b_n:=(2n+1)^{-1}$, which is equivalent to the product of their individual generating functions.
$endgroup$
– Alex R.
1 hour ago
$begingroup$
@MrYouMath: If you write $binom{2n}{n}=binom{2n}{2n-n}$, then the generating function $g(x)$ is the convolution of coefficients $a_n:=binom{2n}{n}^{-1}$ and $b_n:=(2n+1)^{-1}$, which is equivalent to the product of their individual generating functions.
$endgroup$
– Alex R.
1 hour ago
$begingroup$
Hi @MrYouMath, Alex's method is perhaps the best way to discover the generating function. Sometimes, you can even recognize the coefficients to be similar to existing Taylor expansions. In this case, the $a_n b_n$ collectively has similar form to the coefficients in the Taylor series expansion of $sinh^{-1}(x)$. From there, you can apply formal power series operations to try and and match the coefficients. The standard reference is generatingfunctionology by Wilf. Stanley's Enumerative Combinatorics also has an excellent presentation on generating functions / formal power series.
$endgroup$
– Tom Chen
44 mins ago
$begingroup$
Hi @MrYouMath, Alex's method is perhaps the best way to discover the generating function. Sometimes, you can even recognize the coefficients to be similar to existing Taylor expansions. In this case, the $a_n b_n$ collectively has similar form to the coefficients in the Taylor series expansion of $sinh^{-1}(x)$. From there, you can apply formal power series operations to try and and match the coefficients. The standard reference is generatingfunctionology by Wilf. Stanley's Enumerative Combinatorics also has an excellent presentation on generating functions / formal power series.
$endgroup$
– Tom Chen
44 mins ago
add a comment |
coffeee is a new contributor. Be nice, and check out our Code of Conduct.
coffeee is a new contributor. Be nice, and check out our Code of Conduct.
coffeee is a new contributor. Be nice, and check out our Code of Conduct.
coffeee is a new contributor. Be nice, and check out our Code of Conduct.
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6
$begingroup$
Wow, how did you come across this monstrosity?
$endgroup$
– YiFan
3 hours ago