How to show that $sum_{n=1}^{infty}frac{phi^{2n}}{n^2{2n choose n}}=frac{9}{50}pi^2$
$begingroup$
Given:$$sum_{n=1}^{infty}frac{phi^{2n}}{n^2{2n choose n}}=frac{9}{50}pi^2$$
Where $phi=frac{sqrt{5}+1}{2}$
How can I we show that the above sum is correct?
I have checked numerically, it seem correct, but i don't how to proves it
sequences-and-series golden-ratio
$endgroup$
add a comment |
$begingroup$
Given:$$sum_{n=1}^{infty}frac{phi^{2n}}{n^2{2n choose n}}=frac{9}{50}pi^2$$
Where $phi=frac{sqrt{5}+1}{2}$
How can I we show that the above sum is correct?
I have checked numerically, it seem correct, but i don't how to proves it
sequences-and-series golden-ratio
$endgroup$
1
$begingroup$
More generally, $$sum_{n=1}^{infty} frac{x^{2n}}{n^2binom{2n}{n}}=2arcsin^2(x/2)$$
$endgroup$
– projectilemotion
Dec 24 '18 at 10:15
$begingroup$
@Zacky It is right like this. Changing the $pi^2$ to $pi$ would make the equality false.
$endgroup$
– mrtaurho
Dec 24 '18 at 16:01
$begingroup$
You're right! I've made an error when computing myself the sum.
$endgroup$
– Zacky
Dec 24 '18 at 17:13
add a comment |
$begingroup$
Given:$$sum_{n=1}^{infty}frac{phi^{2n}}{n^2{2n choose n}}=frac{9}{50}pi^2$$
Where $phi=frac{sqrt{5}+1}{2}$
How can I we show that the above sum is correct?
I have checked numerically, it seem correct, but i don't how to proves it
sequences-and-series golden-ratio
$endgroup$
Given:$$sum_{n=1}^{infty}frac{phi^{2n}}{n^2{2n choose n}}=frac{9}{50}pi^2$$
Where $phi=frac{sqrt{5}+1}{2}$
How can I we show that the above sum is correct?
I have checked numerically, it seem correct, but i don't how to proves it
sequences-and-series golden-ratio
sequences-and-series golden-ratio
asked Dec 24 '18 at 10:02
user583851user583851
1
1
1
$begingroup$
More generally, $$sum_{n=1}^{infty} frac{x^{2n}}{n^2binom{2n}{n}}=2arcsin^2(x/2)$$
$endgroup$
– projectilemotion
Dec 24 '18 at 10:15
$begingroup$
@Zacky It is right like this. Changing the $pi^2$ to $pi$ would make the equality false.
$endgroup$
– mrtaurho
Dec 24 '18 at 16:01
$begingroup$
You're right! I've made an error when computing myself the sum.
$endgroup$
– Zacky
Dec 24 '18 at 17:13
add a comment |
1
$begingroup$
More generally, $$sum_{n=1}^{infty} frac{x^{2n}}{n^2binom{2n}{n}}=2arcsin^2(x/2)$$
$endgroup$
– projectilemotion
Dec 24 '18 at 10:15
$begingroup$
@Zacky It is right like this. Changing the $pi^2$ to $pi$ would make the equality false.
$endgroup$
– mrtaurho
Dec 24 '18 at 16:01
$begingroup$
You're right! I've made an error when computing myself the sum.
$endgroup$
– Zacky
Dec 24 '18 at 17:13
1
1
$begingroup$
More generally, $$sum_{n=1}^{infty} frac{x^{2n}}{n^2binom{2n}{n}}=2arcsin^2(x/2)$$
$endgroup$
– projectilemotion
Dec 24 '18 at 10:15
$begingroup$
More generally, $$sum_{n=1}^{infty} frac{x^{2n}}{n^2binom{2n}{n}}=2arcsin^2(x/2)$$
$endgroup$
– projectilemotion
Dec 24 '18 at 10:15
$begingroup$
@Zacky It is right like this. Changing the $pi^2$ to $pi$ would make the equality false.
$endgroup$
– mrtaurho
Dec 24 '18 at 16:01
$begingroup$
@Zacky It is right like this. Changing the $pi^2$ to $pi$ would make the equality false.
$endgroup$
– mrtaurho
Dec 24 '18 at 16:01
$begingroup$
You're right! I've made an error when computing myself the sum.
$endgroup$
– Zacky
Dec 24 '18 at 17:13
$begingroup$
You're right! I've made an error when computing myself the sum.
$endgroup$
– Zacky
Dec 24 '18 at 17:13
add a comment |
3 Answers
3
active
oldest
votes
$begingroup$
It can be shown $|x|<4impliessum_{n=1}^inftyfrac{x^n}{n^2binom{2n}{n}}=2arcsin^2frac{sqrt{x}}{2}$, so the choice $x=phi^2$ gives $sum_{n=1}^inftyfrac{phi^{2n}}{n^2binom{2n}{n}}=2arcsin^2frac{phi}{2}=frac{9pi}{50}$ as desired. (The arcsine's evaluation is possible by chasing isosceles triangles in a regular pentagon with inscribed diagonals.)
$endgroup$
add a comment |
$begingroup$
Consider the well-known series expansion of the squared $arcsin$ function. Namely
$$2arcsin^2left(frac x2right)=sum_{n=1}^{infty}frac{x^{2n}}{n^2binom{2n}n}$$
As you can see the RHS equals your given sum for the case that $x=phi$. Plugging this in yields to
$$begin{align}
2arcsin^2left(frac phi2right)=sum_{n=1}^{infty}frac{phi^{2n}}{n^2binom{2n}n}=frac{9pi^2}{50}
end{align}$$
which further reduces the problem to
$$sinleft(frac{3pi}{10}right)=fracphi2=frac{1+sqrt{5}}{4}$$
The latter one can be shown by only using the fact that $cos(pi/5)=1/4(1+sqrt 5)$. It is sufficient to show that
$$begin{align}
sinleft(frac{3pi}{10}right)&=cosleft(frac{pi}5right)\
sinleft(frac{3pi}{10}right)&=cosleft(frac{7pi}{10}-frac{pi}2right)\
sinleft(frac{3pi}{10}right)&=sinleft(frac{7pi}{10}right)\
sinleft(frac{3pi}{10}right)&=sinleft(-frac{3pi}{10}+piright)\
sinleft(frac{3pi}{10}right)&=-sinleft(-frac{3pi}{10}right)
end{align}$$
and the last line turn out to be true since the sine function is odd.
The assumed fact concerning the value of $cos(pi/5)$ can be easily verified by cosidering the fifths roots of unity. To be precise consider
$$begin{align}
x^4+x^3+x^2+x+1&=0\
x^2+frac1{x^2}+x+frac1x+1&=0\
left(x+frac1xright)^2+left(x+frac1xright)-1&=0
end{align}$$
The latter can be solved by the substitution $z=1/x+x$ which boils the problem down to the solving of two quadratic equations. The real part of the first solution $($the one with positive real and imaginary part$)$ equals the cosine of $pi/5$ and we are done.
$endgroup$
add a comment |
$begingroup$
Replace $phi$ with $x$. That gives a function of $x$ with its Taylor series.
Now to find a differential equation that the Taylor series obeys.
Hint:
On one hand, $d^2f/dx^2=sum (2n+2)(2n+1)a_{n+1}x^{2n}$.
On the other hand $xdf/dx=sum 2n a_n x^{2n}$.
Good luck with that. As a second hint, other people have supplied the answer.
$endgroup$
add a comment |
Your Answer
StackExchange.ifUsing("editor", function () {
return StackExchange.using("mathjaxEditing", function () {
StackExchange.MarkdownEditor.creationCallbacks.add(function (editor, postfix) {
StackExchange.mathjaxEditing.prepareWmdForMathJax(editor, postfix, [["$", "$"], ["\\(","\\)"]]);
});
});
}, "mathjax-editing");
StackExchange.ready(function() {
var channelOptions = {
tags: "".split(" "),
id: "69"
};
initTagRenderer("".split(" "), "".split(" "), channelOptions);
StackExchange.using("externalEditor", function() {
// Have to fire editor after snippets, if snippets enabled
if (StackExchange.settings.snippets.snippetsEnabled) {
StackExchange.using("snippets", function() {
createEditor();
});
}
else {
createEditor();
}
});
function createEditor() {
StackExchange.prepareEditor({
heartbeatType: 'answer',
autoActivateHeartbeat: false,
convertImagesToLinks: true,
noModals: true,
showLowRepImageUploadWarning: true,
reputationToPostImages: 10,
bindNavPrevention: true,
postfix: "",
imageUploader: {
brandingHtml: "Powered by u003ca class="icon-imgur-white" href="https://imgur.com/"u003eu003c/au003e",
contentPolicyHtml: "User contributions licensed under u003ca href="https://creativecommons.org/licenses/by-sa/3.0/"u003ecc by-sa 3.0 with attribution requiredu003c/au003e u003ca href="https://stackoverflow.com/legal/content-policy"u003e(content policy)u003c/au003e",
allowUrls: true
},
noCode: true, onDemand: true,
discardSelector: ".discard-answer"
,immediatelyShowMarkdownHelp:true
});
}
});
Sign up or log in
StackExchange.ready(function () {
StackExchange.helpers.onClickDraftSave('#login-link');
});
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Post as a guest
Required, but never shown
StackExchange.ready(
function () {
StackExchange.openid.initPostLogin('.new-post-login', 'https%3a%2f%2fmath.stackexchange.com%2fquestions%2f3051109%2fhow-to-show-that-sum-n-1-infty-frac-phi2nn22n-choose-n-frac9%23new-answer', 'question_page');
}
);
Post as a guest
Required, but never shown
3 Answers
3
active
oldest
votes
3 Answers
3
active
oldest
votes
active
oldest
votes
active
oldest
votes
$begingroup$
It can be shown $|x|<4impliessum_{n=1}^inftyfrac{x^n}{n^2binom{2n}{n}}=2arcsin^2frac{sqrt{x}}{2}$, so the choice $x=phi^2$ gives $sum_{n=1}^inftyfrac{phi^{2n}}{n^2binom{2n}{n}}=2arcsin^2frac{phi}{2}=frac{9pi}{50}$ as desired. (The arcsine's evaluation is possible by chasing isosceles triangles in a regular pentagon with inscribed diagonals.)
$endgroup$
add a comment |
$begingroup$
It can be shown $|x|<4impliessum_{n=1}^inftyfrac{x^n}{n^2binom{2n}{n}}=2arcsin^2frac{sqrt{x}}{2}$, so the choice $x=phi^2$ gives $sum_{n=1}^inftyfrac{phi^{2n}}{n^2binom{2n}{n}}=2arcsin^2frac{phi}{2}=frac{9pi}{50}$ as desired. (The arcsine's evaluation is possible by chasing isosceles triangles in a regular pentagon with inscribed diagonals.)
$endgroup$
add a comment |
$begingroup$
It can be shown $|x|<4impliessum_{n=1}^inftyfrac{x^n}{n^2binom{2n}{n}}=2arcsin^2frac{sqrt{x}}{2}$, so the choice $x=phi^2$ gives $sum_{n=1}^inftyfrac{phi^{2n}}{n^2binom{2n}{n}}=2arcsin^2frac{phi}{2}=frac{9pi}{50}$ as desired. (The arcsine's evaluation is possible by chasing isosceles triangles in a regular pentagon with inscribed diagonals.)
$endgroup$
It can be shown $|x|<4impliessum_{n=1}^inftyfrac{x^n}{n^2binom{2n}{n}}=2arcsin^2frac{sqrt{x}}{2}$, so the choice $x=phi^2$ gives $sum_{n=1}^inftyfrac{phi^{2n}}{n^2binom{2n}{n}}=2arcsin^2frac{phi}{2}=frac{9pi}{50}$ as desired. (The arcsine's evaluation is possible by chasing isosceles triangles in a regular pentagon with inscribed diagonals.)
answered Dec 24 '18 at 10:17
J.G.J.G.
25.7k22540
25.7k22540
add a comment |
add a comment |
$begingroup$
Consider the well-known series expansion of the squared $arcsin$ function. Namely
$$2arcsin^2left(frac x2right)=sum_{n=1}^{infty}frac{x^{2n}}{n^2binom{2n}n}$$
As you can see the RHS equals your given sum for the case that $x=phi$. Plugging this in yields to
$$begin{align}
2arcsin^2left(frac phi2right)=sum_{n=1}^{infty}frac{phi^{2n}}{n^2binom{2n}n}=frac{9pi^2}{50}
end{align}$$
which further reduces the problem to
$$sinleft(frac{3pi}{10}right)=fracphi2=frac{1+sqrt{5}}{4}$$
The latter one can be shown by only using the fact that $cos(pi/5)=1/4(1+sqrt 5)$. It is sufficient to show that
$$begin{align}
sinleft(frac{3pi}{10}right)&=cosleft(frac{pi}5right)\
sinleft(frac{3pi}{10}right)&=cosleft(frac{7pi}{10}-frac{pi}2right)\
sinleft(frac{3pi}{10}right)&=sinleft(frac{7pi}{10}right)\
sinleft(frac{3pi}{10}right)&=sinleft(-frac{3pi}{10}+piright)\
sinleft(frac{3pi}{10}right)&=-sinleft(-frac{3pi}{10}right)
end{align}$$
and the last line turn out to be true since the sine function is odd.
The assumed fact concerning the value of $cos(pi/5)$ can be easily verified by cosidering the fifths roots of unity. To be precise consider
$$begin{align}
x^4+x^3+x^2+x+1&=0\
x^2+frac1{x^2}+x+frac1x+1&=0\
left(x+frac1xright)^2+left(x+frac1xright)-1&=0
end{align}$$
The latter can be solved by the substitution $z=1/x+x$ which boils the problem down to the solving of two quadratic equations. The real part of the first solution $($the one with positive real and imaginary part$)$ equals the cosine of $pi/5$ and we are done.
$endgroup$
add a comment |
$begingroup$
Consider the well-known series expansion of the squared $arcsin$ function. Namely
$$2arcsin^2left(frac x2right)=sum_{n=1}^{infty}frac{x^{2n}}{n^2binom{2n}n}$$
As you can see the RHS equals your given sum for the case that $x=phi$. Plugging this in yields to
$$begin{align}
2arcsin^2left(frac phi2right)=sum_{n=1}^{infty}frac{phi^{2n}}{n^2binom{2n}n}=frac{9pi^2}{50}
end{align}$$
which further reduces the problem to
$$sinleft(frac{3pi}{10}right)=fracphi2=frac{1+sqrt{5}}{4}$$
The latter one can be shown by only using the fact that $cos(pi/5)=1/4(1+sqrt 5)$. It is sufficient to show that
$$begin{align}
sinleft(frac{3pi}{10}right)&=cosleft(frac{pi}5right)\
sinleft(frac{3pi}{10}right)&=cosleft(frac{7pi}{10}-frac{pi}2right)\
sinleft(frac{3pi}{10}right)&=sinleft(frac{7pi}{10}right)\
sinleft(frac{3pi}{10}right)&=sinleft(-frac{3pi}{10}+piright)\
sinleft(frac{3pi}{10}right)&=-sinleft(-frac{3pi}{10}right)
end{align}$$
and the last line turn out to be true since the sine function is odd.
The assumed fact concerning the value of $cos(pi/5)$ can be easily verified by cosidering the fifths roots of unity. To be precise consider
$$begin{align}
x^4+x^3+x^2+x+1&=0\
x^2+frac1{x^2}+x+frac1x+1&=0\
left(x+frac1xright)^2+left(x+frac1xright)-1&=0
end{align}$$
The latter can be solved by the substitution $z=1/x+x$ which boils the problem down to the solving of two quadratic equations. The real part of the first solution $($the one with positive real and imaginary part$)$ equals the cosine of $pi/5$ and we are done.
$endgroup$
add a comment |
$begingroup$
Consider the well-known series expansion of the squared $arcsin$ function. Namely
$$2arcsin^2left(frac x2right)=sum_{n=1}^{infty}frac{x^{2n}}{n^2binom{2n}n}$$
As you can see the RHS equals your given sum for the case that $x=phi$. Plugging this in yields to
$$begin{align}
2arcsin^2left(frac phi2right)=sum_{n=1}^{infty}frac{phi^{2n}}{n^2binom{2n}n}=frac{9pi^2}{50}
end{align}$$
which further reduces the problem to
$$sinleft(frac{3pi}{10}right)=fracphi2=frac{1+sqrt{5}}{4}$$
The latter one can be shown by only using the fact that $cos(pi/5)=1/4(1+sqrt 5)$. It is sufficient to show that
$$begin{align}
sinleft(frac{3pi}{10}right)&=cosleft(frac{pi}5right)\
sinleft(frac{3pi}{10}right)&=cosleft(frac{7pi}{10}-frac{pi}2right)\
sinleft(frac{3pi}{10}right)&=sinleft(frac{7pi}{10}right)\
sinleft(frac{3pi}{10}right)&=sinleft(-frac{3pi}{10}+piright)\
sinleft(frac{3pi}{10}right)&=-sinleft(-frac{3pi}{10}right)
end{align}$$
and the last line turn out to be true since the sine function is odd.
The assumed fact concerning the value of $cos(pi/5)$ can be easily verified by cosidering the fifths roots of unity. To be precise consider
$$begin{align}
x^4+x^3+x^2+x+1&=0\
x^2+frac1{x^2}+x+frac1x+1&=0\
left(x+frac1xright)^2+left(x+frac1xright)-1&=0
end{align}$$
The latter can be solved by the substitution $z=1/x+x$ which boils the problem down to the solving of two quadratic equations. The real part of the first solution $($the one with positive real and imaginary part$)$ equals the cosine of $pi/5$ and we are done.
$endgroup$
Consider the well-known series expansion of the squared $arcsin$ function. Namely
$$2arcsin^2left(frac x2right)=sum_{n=1}^{infty}frac{x^{2n}}{n^2binom{2n}n}$$
As you can see the RHS equals your given sum for the case that $x=phi$. Plugging this in yields to
$$begin{align}
2arcsin^2left(frac phi2right)=sum_{n=1}^{infty}frac{phi^{2n}}{n^2binom{2n}n}=frac{9pi^2}{50}
end{align}$$
which further reduces the problem to
$$sinleft(frac{3pi}{10}right)=fracphi2=frac{1+sqrt{5}}{4}$$
The latter one can be shown by only using the fact that $cos(pi/5)=1/4(1+sqrt 5)$. It is sufficient to show that
$$begin{align}
sinleft(frac{3pi}{10}right)&=cosleft(frac{pi}5right)\
sinleft(frac{3pi}{10}right)&=cosleft(frac{7pi}{10}-frac{pi}2right)\
sinleft(frac{3pi}{10}right)&=sinleft(frac{7pi}{10}right)\
sinleft(frac{3pi}{10}right)&=sinleft(-frac{3pi}{10}+piright)\
sinleft(frac{3pi}{10}right)&=-sinleft(-frac{3pi}{10}right)
end{align}$$
and the last line turn out to be true since the sine function is odd.
The assumed fact concerning the value of $cos(pi/5)$ can be easily verified by cosidering the fifths roots of unity. To be precise consider
$$begin{align}
x^4+x^3+x^2+x+1&=0\
x^2+frac1{x^2}+x+frac1x+1&=0\
left(x+frac1xright)^2+left(x+frac1xright)-1&=0
end{align}$$
The latter can be solved by the substitution $z=1/x+x$ which boils the problem down to the solving of two quadratic equations. The real part of the first solution $($the one with positive real and imaginary part$)$ equals the cosine of $pi/5$ and we are done.
answered Dec 24 '18 at 10:42
mrtaurhomrtaurho
4,55121235
4,55121235
add a comment |
add a comment |
$begingroup$
Replace $phi$ with $x$. That gives a function of $x$ with its Taylor series.
Now to find a differential equation that the Taylor series obeys.
Hint:
On one hand, $d^2f/dx^2=sum (2n+2)(2n+1)a_{n+1}x^{2n}$.
On the other hand $xdf/dx=sum 2n a_n x^{2n}$.
Good luck with that. As a second hint, other people have supplied the answer.
$endgroup$
add a comment |
$begingroup$
Replace $phi$ with $x$. That gives a function of $x$ with its Taylor series.
Now to find a differential equation that the Taylor series obeys.
Hint:
On one hand, $d^2f/dx^2=sum (2n+2)(2n+1)a_{n+1}x^{2n}$.
On the other hand $xdf/dx=sum 2n a_n x^{2n}$.
Good luck with that. As a second hint, other people have supplied the answer.
$endgroup$
add a comment |
$begingroup$
Replace $phi$ with $x$. That gives a function of $x$ with its Taylor series.
Now to find a differential equation that the Taylor series obeys.
Hint:
On one hand, $d^2f/dx^2=sum (2n+2)(2n+1)a_{n+1}x^{2n}$.
On the other hand $xdf/dx=sum 2n a_n x^{2n}$.
Good luck with that. As a second hint, other people have supplied the answer.
$endgroup$
Replace $phi$ with $x$. That gives a function of $x$ with its Taylor series.
Now to find a differential equation that the Taylor series obeys.
Hint:
On one hand, $d^2f/dx^2=sum (2n+2)(2n+1)a_{n+1}x^{2n}$.
On the other hand $xdf/dx=sum 2n a_n x^{2n}$.
Good luck with that. As a second hint, other people have supplied the answer.
answered Dec 24 '18 at 10:40
Empy2Empy2
33.5k12261
33.5k12261
add a comment |
add a comment |
Thanks for contributing an answer to Mathematics Stack Exchange!
- Please be sure to answer the question. Provide details and share your research!
But avoid …
- Asking for help, clarification, or responding to other answers.
- Making statements based on opinion; back them up with references or personal experience.
Use MathJax to format equations. MathJax reference.
To learn more, see our tips on writing great answers.
Sign up or log in
StackExchange.ready(function () {
StackExchange.helpers.onClickDraftSave('#login-link');
});
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Post as a guest
Required, but never shown
StackExchange.ready(
function () {
StackExchange.openid.initPostLogin('.new-post-login', 'https%3a%2f%2fmath.stackexchange.com%2fquestions%2f3051109%2fhow-to-show-that-sum-n-1-infty-frac-phi2nn22n-choose-n-frac9%23new-answer', 'question_page');
}
);
Post as a guest
Required, but never shown
Sign up or log in
StackExchange.ready(function () {
StackExchange.helpers.onClickDraftSave('#login-link');
});
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Post as a guest
Required, but never shown
Sign up or log in
StackExchange.ready(function () {
StackExchange.helpers.onClickDraftSave('#login-link');
});
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Post as a guest
Required, but never shown
Sign up or log in
StackExchange.ready(function () {
StackExchange.helpers.onClickDraftSave('#login-link');
});
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Post as a guest
Required, but never shown
Required, but never shown
Required, but never shown
Required, but never shown
Required, but never shown
Required, but never shown
Required, but never shown
Required, but never shown
Required, but never shown
1
$begingroup$
More generally, $$sum_{n=1}^{infty} frac{x^{2n}}{n^2binom{2n}{n}}=2arcsin^2(x/2)$$
$endgroup$
– projectilemotion
Dec 24 '18 at 10:15
$begingroup$
@Zacky It is right like this. Changing the $pi^2$ to $pi$ would make the equality false.
$endgroup$
– mrtaurho
Dec 24 '18 at 16:01
$begingroup$
You're right! I've made an error when computing myself the sum.
$endgroup$
– Zacky
Dec 24 '18 at 17:13