How does this infinite series simplify to an integral?
$begingroup$
How does the infinite series in the link above simplify to that integral?
$$1-frac{1}{4}+frac{1}{7}-frac{1}{10}+cdots=int_0^1frac{dx}{1+x^3}$$
I thought of simplifying the series to the sum to infinity of $frac{1}{6n-5} - frac{1}{6n-2}$, but this did not help.
integration sequences-and-series power-series improper-integrals
$endgroup$
add a comment |
$begingroup$
How does the infinite series in the link above simplify to that integral?
$$1-frac{1}{4}+frac{1}{7}-frac{1}{10}+cdots=int_0^1frac{dx}{1+x^3}$$
I thought of simplifying the series to the sum to infinity of $frac{1}{6n-5} - frac{1}{6n-2}$, but this did not help.
integration sequences-and-series power-series improper-integrals
$endgroup$
1
$begingroup$
en.wikipedia.org/wiki/Binomial_series $$(1+x^3)^{-1}$$
$endgroup$
– lab bhattacharjee
3 hours ago
$begingroup$
It can be obtained by integrating term by term the infinite series expansion of $(1+x^3)^{-1}$.
$endgroup$
– StubbornAtom
3 hours ago
add a comment |
$begingroup$
How does the infinite series in the link above simplify to that integral?
$$1-frac{1}{4}+frac{1}{7}-frac{1}{10}+cdots=int_0^1frac{dx}{1+x^3}$$
I thought of simplifying the series to the sum to infinity of $frac{1}{6n-5} - frac{1}{6n-2}$, but this did not help.
integration sequences-and-series power-series improper-integrals
$endgroup$
How does the infinite series in the link above simplify to that integral?
$$1-frac{1}{4}+frac{1}{7}-frac{1}{10}+cdots=int_0^1frac{dx}{1+x^3}$$
I thought of simplifying the series to the sum to infinity of $frac{1}{6n-5} - frac{1}{6n-2}$, but this did not help.
integration sequences-and-series power-series improper-integrals
integration sequences-and-series power-series improper-integrals
edited 3 hours ago
Zacky
7,87511062
7,87511062
asked 3 hours ago
ShreeShree
114
114
1
$begingroup$
en.wikipedia.org/wiki/Binomial_series $$(1+x^3)^{-1}$$
$endgroup$
– lab bhattacharjee
3 hours ago
$begingroup$
It can be obtained by integrating term by term the infinite series expansion of $(1+x^3)^{-1}$.
$endgroup$
– StubbornAtom
3 hours ago
add a comment |
1
$begingroup$
en.wikipedia.org/wiki/Binomial_series $$(1+x^3)^{-1}$$
$endgroup$
– lab bhattacharjee
3 hours ago
$begingroup$
It can be obtained by integrating term by term the infinite series expansion of $(1+x^3)^{-1}$.
$endgroup$
– StubbornAtom
3 hours ago
1
1
$begingroup$
en.wikipedia.org/wiki/Binomial_series $$(1+x^3)^{-1}$$
$endgroup$
– lab bhattacharjee
3 hours ago
$begingroup$
en.wikipedia.org/wiki/Binomial_series $$(1+x^3)^{-1}$$
$endgroup$
– lab bhattacharjee
3 hours ago
$begingroup$
It can be obtained by integrating term by term the infinite series expansion of $(1+x^3)^{-1}$.
$endgroup$
– StubbornAtom
3 hours ago
$begingroup$
It can be obtained by integrating term by term the infinite series expansion of $(1+x^3)^{-1}$.
$endgroup$
– StubbornAtom
3 hours ago
add a comment |
3 Answers
3
active
oldest
votes
$begingroup$
$$int_{0}^{1}{frac{1}{1-(-x)^3}=int_{0}^{1}{sum_{n=0}^{infty}{(-x)}^{3n}}}dx=sum_{n=0}^{infty}{(-1)^{3n}int_{0}^{1}{x^{3n}}dx}$$
$$=sum_{n=0}^{infty}{frac{(-1)^{3n}}{3n+1}}= 1-frac{1}{4}+frac{1}{7}-frac{1}{10}+cdots $$
$endgroup$
$begingroup$
For those who aren't clear on the first equality, it's a geometric series formula.
$endgroup$
– Alexis Olson
18 mins ago
add a comment |
$begingroup$
for $x$ real, $ngeq 0$ integer
begin{align}frac{1}{1+x^3}&=frac{1-(-x^3)^{n+1}}{1-(-x^3)}+frac{(-x^3)^{n+1}}{1-(-x^3)}\
&=frac{1-(-x^3)^{n+1}}{1-(-x^3)}+frac{(-x^3)^{n+1}}{1+x^3}\
end{align}
For $xneq 1$, $ngeq 0$ integer, begin{align}sum_{k=0}^n x^k=frac{1-x^{n+1}}{1-x}end{align}
Therefore,
begin{align}int_0^1 frac{1}{1+x^3},dx&=int_0^1 left(sum_{k=0}^n (-x^3)^kright),dx+int_0^1 frac{(-x^3)^{n+1}}{1+x^3},dx\
&=sum_{k=0}^n left(int_0^1 (-x^3)^k,dxright)+int_0^1 frac{(-x^3)^{n+1}}{1+x^3},dx\
&=sum_{k=0}^n frac{(-1)^k}{3k+1}+int_0^1 frac{(-x^3)^{n+1}}{1+x^3},dx\
end{align}
For $xin[0;1],ngeq 0$, integer,
begin{align}frac{x^{3(n+1)}}{1+x^3}leq x^{3(n+1)}end{align}
and,
begin{align}int_0^1 x^{3(n+1)},dx=frac{1}{3n+4}end{align}
Therefore,
begin{align}left|int_0^1 frac{(-x^3)^{n+1}}{1+x^3},dxright|leq frac{1}{3n+4}end{align}
begin{align}left|int_0^1 frac{1}{1+x^3},dx-sum_{k=0}^n frac{(-1)^k}{3k+1}right|leq frac{1}{3n+4}end{align}
Therefore,
begin{align}boxed{int_0^1 frac{1}{1+x^3},dx=sum_{k=0}^infty frac{(-1)^k}{3k+1}}end{align}
$endgroup$
add a comment |
$begingroup$
If $lvert xrvert<1$, let$$f(x)=sum_{n=0}^inftyfrac{x^{3n+1}}{3n+1}.$$Then $$f'(x)=sum_{n=0}^infty x^{3n}=frac1{1-x^3}.$$Thereforebegin{align}1-frac14+frac17-frac1{10}+cdots&=lim_{xto1}f(x)\&=int_0^1f'(x),mathrm dx\&=int_0^1frac1{1-x^3},mathrm dx.end{align}
$endgroup$
add a comment |
Your Answer
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%2f3186397%2fhow-does-this-infinite-series-simplify-to-an-integral%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
$begingroup$
$$int_{0}^{1}{frac{1}{1-(-x)^3}=int_{0}^{1}{sum_{n=0}^{infty}{(-x)}^{3n}}}dx=sum_{n=0}^{infty}{(-1)^{3n}int_{0}^{1}{x^{3n}}dx}$$
$$=sum_{n=0}^{infty}{frac{(-1)^{3n}}{3n+1}}= 1-frac{1}{4}+frac{1}{7}-frac{1}{10}+cdots $$
$endgroup$
$begingroup$
For those who aren't clear on the first equality, it's a geometric series formula.
$endgroup$
– Alexis Olson
18 mins ago
add a comment |
$begingroup$
$$int_{0}^{1}{frac{1}{1-(-x)^3}=int_{0}^{1}{sum_{n=0}^{infty}{(-x)}^{3n}}}dx=sum_{n=0}^{infty}{(-1)^{3n}int_{0}^{1}{x^{3n}}dx}$$
$$=sum_{n=0}^{infty}{frac{(-1)^{3n}}{3n+1}}= 1-frac{1}{4}+frac{1}{7}-frac{1}{10}+cdots $$
$endgroup$
$begingroup$
For those who aren't clear on the first equality, it's a geometric series formula.
$endgroup$
– Alexis Olson
18 mins ago
add a comment |
$begingroup$
$$int_{0}^{1}{frac{1}{1-(-x)^3}=int_{0}^{1}{sum_{n=0}^{infty}{(-x)}^{3n}}}dx=sum_{n=0}^{infty}{(-1)^{3n}int_{0}^{1}{x^{3n}}dx}$$
$$=sum_{n=0}^{infty}{frac{(-1)^{3n}}{3n+1}}= 1-frac{1}{4}+frac{1}{7}-frac{1}{10}+cdots $$
$endgroup$
$$int_{0}^{1}{frac{1}{1-(-x)^3}=int_{0}^{1}{sum_{n=0}^{infty}{(-x)}^{3n}}}dx=sum_{n=0}^{infty}{(-1)^{3n}int_{0}^{1}{x^{3n}}dx}$$
$$=sum_{n=0}^{infty}{frac{(-1)^{3n}}{3n+1}}= 1-frac{1}{4}+frac{1}{7}-frac{1}{10}+cdots $$
answered 3 hours ago
HAMIDINE SOUMAREHAMIDINE SOUMARE
2,184212
2,184212
$begingroup$
For those who aren't clear on the first equality, it's a geometric series formula.
$endgroup$
– Alexis Olson
18 mins ago
add a comment |
$begingroup$
For those who aren't clear on the first equality, it's a geometric series formula.
$endgroup$
– Alexis Olson
18 mins ago
$begingroup$
For those who aren't clear on the first equality, it's a geometric series formula.
$endgroup$
– Alexis Olson
18 mins ago
$begingroup$
For those who aren't clear on the first equality, it's a geometric series formula.
$endgroup$
– Alexis Olson
18 mins ago
add a comment |
$begingroup$
for $x$ real, $ngeq 0$ integer
begin{align}frac{1}{1+x^3}&=frac{1-(-x^3)^{n+1}}{1-(-x^3)}+frac{(-x^3)^{n+1}}{1-(-x^3)}\
&=frac{1-(-x^3)^{n+1}}{1-(-x^3)}+frac{(-x^3)^{n+1}}{1+x^3}\
end{align}
For $xneq 1$, $ngeq 0$ integer, begin{align}sum_{k=0}^n x^k=frac{1-x^{n+1}}{1-x}end{align}
Therefore,
begin{align}int_0^1 frac{1}{1+x^3},dx&=int_0^1 left(sum_{k=0}^n (-x^3)^kright),dx+int_0^1 frac{(-x^3)^{n+1}}{1+x^3},dx\
&=sum_{k=0}^n left(int_0^1 (-x^3)^k,dxright)+int_0^1 frac{(-x^3)^{n+1}}{1+x^3},dx\
&=sum_{k=0}^n frac{(-1)^k}{3k+1}+int_0^1 frac{(-x^3)^{n+1}}{1+x^3},dx\
end{align}
For $xin[0;1],ngeq 0$, integer,
begin{align}frac{x^{3(n+1)}}{1+x^3}leq x^{3(n+1)}end{align}
and,
begin{align}int_0^1 x^{3(n+1)},dx=frac{1}{3n+4}end{align}
Therefore,
begin{align}left|int_0^1 frac{(-x^3)^{n+1}}{1+x^3},dxright|leq frac{1}{3n+4}end{align}
begin{align}left|int_0^1 frac{1}{1+x^3},dx-sum_{k=0}^n frac{(-1)^k}{3k+1}right|leq frac{1}{3n+4}end{align}
Therefore,
begin{align}boxed{int_0^1 frac{1}{1+x^3},dx=sum_{k=0}^infty frac{(-1)^k}{3k+1}}end{align}
$endgroup$
add a comment |
$begingroup$
for $x$ real, $ngeq 0$ integer
begin{align}frac{1}{1+x^3}&=frac{1-(-x^3)^{n+1}}{1-(-x^3)}+frac{(-x^3)^{n+1}}{1-(-x^3)}\
&=frac{1-(-x^3)^{n+1}}{1-(-x^3)}+frac{(-x^3)^{n+1}}{1+x^3}\
end{align}
For $xneq 1$, $ngeq 0$ integer, begin{align}sum_{k=0}^n x^k=frac{1-x^{n+1}}{1-x}end{align}
Therefore,
begin{align}int_0^1 frac{1}{1+x^3},dx&=int_0^1 left(sum_{k=0}^n (-x^3)^kright),dx+int_0^1 frac{(-x^3)^{n+1}}{1+x^3},dx\
&=sum_{k=0}^n left(int_0^1 (-x^3)^k,dxright)+int_0^1 frac{(-x^3)^{n+1}}{1+x^3},dx\
&=sum_{k=0}^n frac{(-1)^k}{3k+1}+int_0^1 frac{(-x^3)^{n+1}}{1+x^3},dx\
end{align}
For $xin[0;1],ngeq 0$, integer,
begin{align}frac{x^{3(n+1)}}{1+x^3}leq x^{3(n+1)}end{align}
and,
begin{align}int_0^1 x^{3(n+1)},dx=frac{1}{3n+4}end{align}
Therefore,
begin{align}left|int_0^1 frac{(-x^3)^{n+1}}{1+x^3},dxright|leq frac{1}{3n+4}end{align}
begin{align}left|int_0^1 frac{1}{1+x^3},dx-sum_{k=0}^n frac{(-1)^k}{3k+1}right|leq frac{1}{3n+4}end{align}
Therefore,
begin{align}boxed{int_0^1 frac{1}{1+x^3},dx=sum_{k=0}^infty frac{(-1)^k}{3k+1}}end{align}
$endgroup$
add a comment |
$begingroup$
for $x$ real, $ngeq 0$ integer
begin{align}frac{1}{1+x^3}&=frac{1-(-x^3)^{n+1}}{1-(-x^3)}+frac{(-x^3)^{n+1}}{1-(-x^3)}\
&=frac{1-(-x^3)^{n+1}}{1-(-x^3)}+frac{(-x^3)^{n+1}}{1+x^3}\
end{align}
For $xneq 1$, $ngeq 0$ integer, begin{align}sum_{k=0}^n x^k=frac{1-x^{n+1}}{1-x}end{align}
Therefore,
begin{align}int_0^1 frac{1}{1+x^3},dx&=int_0^1 left(sum_{k=0}^n (-x^3)^kright),dx+int_0^1 frac{(-x^3)^{n+1}}{1+x^3},dx\
&=sum_{k=0}^n left(int_0^1 (-x^3)^k,dxright)+int_0^1 frac{(-x^3)^{n+1}}{1+x^3},dx\
&=sum_{k=0}^n frac{(-1)^k}{3k+1}+int_0^1 frac{(-x^3)^{n+1}}{1+x^3},dx\
end{align}
For $xin[0;1],ngeq 0$, integer,
begin{align}frac{x^{3(n+1)}}{1+x^3}leq x^{3(n+1)}end{align}
and,
begin{align}int_0^1 x^{3(n+1)},dx=frac{1}{3n+4}end{align}
Therefore,
begin{align}left|int_0^1 frac{(-x^3)^{n+1}}{1+x^3},dxright|leq frac{1}{3n+4}end{align}
begin{align}left|int_0^1 frac{1}{1+x^3},dx-sum_{k=0}^n frac{(-1)^k}{3k+1}right|leq frac{1}{3n+4}end{align}
Therefore,
begin{align}boxed{int_0^1 frac{1}{1+x^3},dx=sum_{k=0}^infty frac{(-1)^k}{3k+1}}end{align}
$endgroup$
for $x$ real, $ngeq 0$ integer
begin{align}frac{1}{1+x^3}&=frac{1-(-x^3)^{n+1}}{1-(-x^3)}+frac{(-x^3)^{n+1}}{1-(-x^3)}\
&=frac{1-(-x^3)^{n+1}}{1-(-x^3)}+frac{(-x^3)^{n+1}}{1+x^3}\
end{align}
For $xneq 1$, $ngeq 0$ integer, begin{align}sum_{k=0}^n x^k=frac{1-x^{n+1}}{1-x}end{align}
Therefore,
begin{align}int_0^1 frac{1}{1+x^3},dx&=int_0^1 left(sum_{k=0}^n (-x^3)^kright),dx+int_0^1 frac{(-x^3)^{n+1}}{1+x^3},dx\
&=sum_{k=0}^n left(int_0^1 (-x^3)^k,dxright)+int_0^1 frac{(-x^3)^{n+1}}{1+x^3},dx\
&=sum_{k=0}^n frac{(-1)^k}{3k+1}+int_0^1 frac{(-x^3)^{n+1}}{1+x^3},dx\
end{align}
For $xin[0;1],ngeq 0$, integer,
begin{align}frac{x^{3(n+1)}}{1+x^3}leq x^{3(n+1)}end{align}
and,
begin{align}int_0^1 x^{3(n+1)},dx=frac{1}{3n+4}end{align}
Therefore,
begin{align}left|int_0^1 frac{(-x^3)^{n+1}}{1+x^3},dxright|leq frac{1}{3n+4}end{align}
begin{align}left|int_0^1 frac{1}{1+x^3},dx-sum_{k=0}^n frac{(-1)^k}{3k+1}right|leq frac{1}{3n+4}end{align}
Therefore,
begin{align}boxed{int_0^1 frac{1}{1+x^3},dx=sum_{k=0}^infty frac{(-1)^k}{3k+1}}end{align}
answered 3 hours ago
FDPFDP
6,12211929
6,12211929
add a comment |
add a comment |
$begingroup$
If $lvert xrvert<1$, let$$f(x)=sum_{n=0}^inftyfrac{x^{3n+1}}{3n+1}.$$Then $$f'(x)=sum_{n=0}^infty x^{3n}=frac1{1-x^3}.$$Thereforebegin{align}1-frac14+frac17-frac1{10}+cdots&=lim_{xto1}f(x)\&=int_0^1f'(x),mathrm dx\&=int_0^1frac1{1-x^3},mathrm dx.end{align}
$endgroup$
add a comment |
$begingroup$
If $lvert xrvert<1$, let$$f(x)=sum_{n=0}^inftyfrac{x^{3n+1}}{3n+1}.$$Then $$f'(x)=sum_{n=0}^infty x^{3n}=frac1{1-x^3}.$$Thereforebegin{align}1-frac14+frac17-frac1{10}+cdots&=lim_{xto1}f(x)\&=int_0^1f'(x),mathrm dx\&=int_0^1frac1{1-x^3},mathrm dx.end{align}
$endgroup$
add a comment |
$begingroup$
If $lvert xrvert<1$, let$$f(x)=sum_{n=0}^inftyfrac{x^{3n+1}}{3n+1}.$$Then $$f'(x)=sum_{n=0}^infty x^{3n}=frac1{1-x^3}.$$Thereforebegin{align}1-frac14+frac17-frac1{10}+cdots&=lim_{xto1}f(x)\&=int_0^1f'(x),mathrm dx\&=int_0^1frac1{1-x^3},mathrm dx.end{align}
$endgroup$
If $lvert xrvert<1$, let$$f(x)=sum_{n=0}^inftyfrac{x^{3n+1}}{3n+1}.$$Then $$f'(x)=sum_{n=0}^infty x^{3n}=frac1{1-x^3}.$$Thereforebegin{align}1-frac14+frac17-frac1{10}+cdots&=lim_{xto1}f(x)\&=int_0^1f'(x),mathrm dx\&=int_0^1frac1{1-x^3},mathrm dx.end{align}
answered 3 hours ago
José Carlos SantosJosé Carlos Santos
174k23133243
174k23133243
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%2f3186397%2fhow-does-this-infinite-series-simplify-to-an-integral%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
$begingroup$
en.wikipedia.org/wiki/Binomial_series $$(1+x^3)^{-1}$$
$endgroup$
– lab bhattacharjee
3 hours ago
$begingroup$
It can be obtained by integrating term by term the infinite series expansion of $(1+x^3)^{-1}$.
$endgroup$
– StubbornAtom
3 hours ago