A Series for $pi$
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Question: Can we show that $$sum_{n=0}^infty(-1)^{n+1}frac{(2n-3)!!}{(2n+3)!!}=frac{pi}{8} $$ ?
According to wolfram alpha this result is true.
Just amateur curiosity, not sure of a starting point to show if this true.
calculus sequences-and-series pi
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add a comment |
$begingroup$
Question: Can we show that $$sum_{n=0}^infty(-1)^{n+1}frac{(2n-3)!!}{(2n+3)!!}=frac{pi}{8} $$ ?
According to wolfram alpha this result is true.
Just amateur curiosity, not sure of a starting point to show if this true.
calculus sequences-and-series pi
$endgroup$
$begingroup$
For my own amusement, I will choose to believe this user is actually Sarah Silverman, the actress and comedian.
$endgroup$
– The Count
Dec 30 '18 at 23:36
add a comment |
$begingroup$
Question: Can we show that $$sum_{n=0}^infty(-1)^{n+1}frac{(2n-3)!!}{(2n+3)!!}=frac{pi}{8} $$ ?
According to wolfram alpha this result is true.
Just amateur curiosity, not sure of a starting point to show if this true.
calculus sequences-and-series pi
$endgroup$
Question: Can we show that $$sum_{n=0}^infty(-1)^{n+1}frac{(2n-3)!!}{(2n+3)!!}=frac{pi}{8} $$ ?
According to wolfram alpha this result is true.
Just amateur curiosity, not sure of a starting point to show if this true.
calculus sequences-and-series pi
calculus sequences-and-series pi
asked Dec 30 '18 at 23:18
S. SilvermanS. Silverman
497
497
$begingroup$
For my own amusement, I will choose to believe this user is actually Sarah Silverman, the actress and comedian.
$endgroup$
– The Count
Dec 30 '18 at 23:36
add a comment |
$begingroup$
For my own amusement, I will choose to believe this user is actually Sarah Silverman, the actress and comedian.
$endgroup$
– The Count
Dec 30 '18 at 23:36
$begingroup$
For my own amusement, I will choose to believe this user is actually Sarah Silverman, the actress and comedian.
$endgroup$
– The Count
Dec 30 '18 at 23:36
$begingroup$
For my own amusement, I will choose to believe this user is actually Sarah Silverman, the actress and comedian.
$endgroup$
– The Count
Dec 30 '18 at 23:36
add a comment |
2 Answers
2
active
oldest
votes
$begingroup$
Note that after cancellation we have
$$frac{(2n-3)!!}{(2n+3)!!}=frac{1}{(2n-1)(2n+1)(2n+3)}$$
Taking partial fractions gives
$$
frac{1}{(2n-1)(2n+1)(2n+3)}=frac{1}{8}left(-frac{1}{2n-1}+frac{2}{2n+1}-frac{1}{2n+3}right)
$$
Now, the standard arctangent series for $pi$ gives $sum_{n=0}^infty (-1)^n frac{1}{2n+1}=frac{pi}{4}$. So
$$sum_{n=0}^infty (-1)^n frac{1}{2n-1}=-sum_{n=-1}^infty (-1)^n frac{1}{2n+1}=-1-frac{pi}{4}$$
$$
sum_{n=0}^infty (-1)^n frac{1}{2n-3}=-sum_{n=1}^infty (-1)^n frac{1}{2n+1}=1-frac{pi}{4}
$$
which means that when we combine all three infinite sums we get
$$
frac{1}{8}left[-left(-1-frac{pi}{4}right)+2left(frac{pi}{4}right)-left(1-frac{pi}{4}right)right]=frac{pi}{8}
$$
as desired.
(This solution makes it obvious that the sum should be of the form $a+bpi$ with $a$ and $b$ rational, but makes the cancellation of the rational term look somewhat coincidental. I would be curious to know if there's a different way of doing it which more clearly demonstrates why the sum ends up being a rational multiple of $pi$.)
$endgroup$
add a comment |
$begingroup$
Considering the simplicity of Micah's answer, I am ashamed to provide this complex one.
Let
$$S_p(x)=sum_{n=0}^p(-1)^{n+1}frac{(2n-3)!!}{(2n+3)!!}x^p$$ It write
$$S_p(x)=frac{left(8 p^3+36 p^2+46 p+15right) ,
_2F_1left(-frac{1}{2},1;frac{5}{2};-xright)+3 (-x)^{p+1} ,
_2F_1left(1,p+frac{1}{2};p+frac{7}{2};-xright)}{3 left(8 p^3+36 p^2+46
p+15right)}$$ Using
$$,
_2F_1left(-frac{1}{2},1;frac{5}{2};-1right)=frac{3 pi }{8}$$ then
$$S_p(1)=frac{pi left(8 p^3+36 p^2+46 p+15right)-8 (-1)^p ,
_2F_1left(1,p+frac{1}{2};p+frac{7}{2};-1right)}{8 left(8 p^3+36 p^2+46
p+15right)}$$ that it to say
$$S_p(1)=frac pi 8-frac{(-1)^p }{8 p^3+36
p^2+46 p+15}, _2F_1left(1,p+frac{1}{2};p+frac{7}{2};-1right)$$ and $, _2F_1left(1,p+frac{1}{2};p+frac{7}{2};-1right)$ looks like an hyperbolic function going asymptotically to $frac 12$.
Then $S_infty(1)=frac{ pi }{8}$.
$endgroup$
add a comment |
Your Answer
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2 Answers
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active
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2 Answers
2
active
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$begingroup$
Note that after cancellation we have
$$frac{(2n-3)!!}{(2n+3)!!}=frac{1}{(2n-1)(2n+1)(2n+3)}$$
Taking partial fractions gives
$$
frac{1}{(2n-1)(2n+1)(2n+3)}=frac{1}{8}left(-frac{1}{2n-1}+frac{2}{2n+1}-frac{1}{2n+3}right)
$$
Now, the standard arctangent series for $pi$ gives $sum_{n=0}^infty (-1)^n frac{1}{2n+1}=frac{pi}{4}$. So
$$sum_{n=0}^infty (-1)^n frac{1}{2n-1}=-sum_{n=-1}^infty (-1)^n frac{1}{2n+1}=-1-frac{pi}{4}$$
$$
sum_{n=0}^infty (-1)^n frac{1}{2n-3}=-sum_{n=1}^infty (-1)^n frac{1}{2n+1}=1-frac{pi}{4}
$$
which means that when we combine all three infinite sums we get
$$
frac{1}{8}left[-left(-1-frac{pi}{4}right)+2left(frac{pi}{4}right)-left(1-frac{pi}{4}right)right]=frac{pi}{8}
$$
as desired.
(This solution makes it obvious that the sum should be of the form $a+bpi$ with $a$ and $b$ rational, but makes the cancellation of the rational term look somewhat coincidental. I would be curious to know if there's a different way of doing it which more clearly demonstrates why the sum ends up being a rational multiple of $pi$.)
$endgroup$
add a comment |
$begingroup$
Note that after cancellation we have
$$frac{(2n-3)!!}{(2n+3)!!}=frac{1}{(2n-1)(2n+1)(2n+3)}$$
Taking partial fractions gives
$$
frac{1}{(2n-1)(2n+1)(2n+3)}=frac{1}{8}left(-frac{1}{2n-1}+frac{2}{2n+1}-frac{1}{2n+3}right)
$$
Now, the standard arctangent series for $pi$ gives $sum_{n=0}^infty (-1)^n frac{1}{2n+1}=frac{pi}{4}$. So
$$sum_{n=0}^infty (-1)^n frac{1}{2n-1}=-sum_{n=-1}^infty (-1)^n frac{1}{2n+1}=-1-frac{pi}{4}$$
$$
sum_{n=0}^infty (-1)^n frac{1}{2n-3}=-sum_{n=1}^infty (-1)^n frac{1}{2n+1}=1-frac{pi}{4}
$$
which means that when we combine all three infinite sums we get
$$
frac{1}{8}left[-left(-1-frac{pi}{4}right)+2left(frac{pi}{4}right)-left(1-frac{pi}{4}right)right]=frac{pi}{8}
$$
as desired.
(This solution makes it obvious that the sum should be of the form $a+bpi$ with $a$ and $b$ rational, but makes the cancellation of the rational term look somewhat coincidental. I would be curious to know if there's a different way of doing it which more clearly demonstrates why the sum ends up being a rational multiple of $pi$.)
$endgroup$
add a comment |
$begingroup$
Note that after cancellation we have
$$frac{(2n-3)!!}{(2n+3)!!}=frac{1}{(2n-1)(2n+1)(2n+3)}$$
Taking partial fractions gives
$$
frac{1}{(2n-1)(2n+1)(2n+3)}=frac{1}{8}left(-frac{1}{2n-1}+frac{2}{2n+1}-frac{1}{2n+3}right)
$$
Now, the standard arctangent series for $pi$ gives $sum_{n=0}^infty (-1)^n frac{1}{2n+1}=frac{pi}{4}$. So
$$sum_{n=0}^infty (-1)^n frac{1}{2n-1}=-sum_{n=-1}^infty (-1)^n frac{1}{2n+1}=-1-frac{pi}{4}$$
$$
sum_{n=0}^infty (-1)^n frac{1}{2n-3}=-sum_{n=1}^infty (-1)^n frac{1}{2n+1}=1-frac{pi}{4}
$$
which means that when we combine all three infinite sums we get
$$
frac{1}{8}left[-left(-1-frac{pi}{4}right)+2left(frac{pi}{4}right)-left(1-frac{pi}{4}right)right]=frac{pi}{8}
$$
as desired.
(This solution makes it obvious that the sum should be of the form $a+bpi$ with $a$ and $b$ rational, but makes the cancellation of the rational term look somewhat coincidental. I would be curious to know if there's a different way of doing it which more clearly demonstrates why the sum ends up being a rational multiple of $pi$.)
$endgroup$
Note that after cancellation we have
$$frac{(2n-3)!!}{(2n+3)!!}=frac{1}{(2n-1)(2n+1)(2n+3)}$$
Taking partial fractions gives
$$
frac{1}{(2n-1)(2n+1)(2n+3)}=frac{1}{8}left(-frac{1}{2n-1}+frac{2}{2n+1}-frac{1}{2n+3}right)
$$
Now, the standard arctangent series for $pi$ gives $sum_{n=0}^infty (-1)^n frac{1}{2n+1}=frac{pi}{4}$. So
$$sum_{n=0}^infty (-1)^n frac{1}{2n-1}=-sum_{n=-1}^infty (-1)^n frac{1}{2n+1}=-1-frac{pi}{4}$$
$$
sum_{n=0}^infty (-1)^n frac{1}{2n-3}=-sum_{n=1}^infty (-1)^n frac{1}{2n+1}=1-frac{pi}{4}
$$
which means that when we combine all three infinite sums we get
$$
frac{1}{8}left[-left(-1-frac{pi}{4}right)+2left(frac{pi}{4}right)-left(1-frac{pi}{4}right)right]=frac{pi}{8}
$$
as desired.
(This solution makes it obvious that the sum should be of the form $a+bpi$ with $a$ and $b$ rational, but makes the cancellation of the rational term look somewhat coincidental. I would be curious to know if there's a different way of doing it which more clearly demonstrates why the sum ends up being a rational multiple of $pi$.)
edited Dec 30 '18 at 23:37
answered Dec 30 '18 at 23:31
MicahMicah
30.2k1364106
30.2k1364106
add a comment |
add a comment |
$begingroup$
Considering the simplicity of Micah's answer, I am ashamed to provide this complex one.
Let
$$S_p(x)=sum_{n=0}^p(-1)^{n+1}frac{(2n-3)!!}{(2n+3)!!}x^p$$ It write
$$S_p(x)=frac{left(8 p^3+36 p^2+46 p+15right) ,
_2F_1left(-frac{1}{2},1;frac{5}{2};-xright)+3 (-x)^{p+1} ,
_2F_1left(1,p+frac{1}{2};p+frac{7}{2};-xright)}{3 left(8 p^3+36 p^2+46
p+15right)}$$ Using
$$,
_2F_1left(-frac{1}{2},1;frac{5}{2};-1right)=frac{3 pi }{8}$$ then
$$S_p(1)=frac{pi left(8 p^3+36 p^2+46 p+15right)-8 (-1)^p ,
_2F_1left(1,p+frac{1}{2};p+frac{7}{2};-1right)}{8 left(8 p^3+36 p^2+46
p+15right)}$$ that it to say
$$S_p(1)=frac pi 8-frac{(-1)^p }{8 p^3+36
p^2+46 p+15}, _2F_1left(1,p+frac{1}{2};p+frac{7}{2};-1right)$$ and $, _2F_1left(1,p+frac{1}{2};p+frac{7}{2};-1right)$ looks like an hyperbolic function going asymptotically to $frac 12$.
Then $S_infty(1)=frac{ pi }{8}$.
$endgroup$
add a comment |
$begingroup$
Considering the simplicity of Micah's answer, I am ashamed to provide this complex one.
Let
$$S_p(x)=sum_{n=0}^p(-1)^{n+1}frac{(2n-3)!!}{(2n+3)!!}x^p$$ It write
$$S_p(x)=frac{left(8 p^3+36 p^2+46 p+15right) ,
_2F_1left(-frac{1}{2},1;frac{5}{2};-xright)+3 (-x)^{p+1} ,
_2F_1left(1,p+frac{1}{2};p+frac{7}{2};-xright)}{3 left(8 p^3+36 p^2+46
p+15right)}$$ Using
$$,
_2F_1left(-frac{1}{2},1;frac{5}{2};-1right)=frac{3 pi }{8}$$ then
$$S_p(1)=frac{pi left(8 p^3+36 p^2+46 p+15right)-8 (-1)^p ,
_2F_1left(1,p+frac{1}{2};p+frac{7}{2};-1right)}{8 left(8 p^3+36 p^2+46
p+15right)}$$ that it to say
$$S_p(1)=frac pi 8-frac{(-1)^p }{8 p^3+36
p^2+46 p+15}, _2F_1left(1,p+frac{1}{2};p+frac{7}{2};-1right)$$ and $, _2F_1left(1,p+frac{1}{2};p+frac{7}{2};-1right)$ looks like an hyperbolic function going asymptotically to $frac 12$.
Then $S_infty(1)=frac{ pi }{8}$.
$endgroup$
add a comment |
$begingroup$
Considering the simplicity of Micah's answer, I am ashamed to provide this complex one.
Let
$$S_p(x)=sum_{n=0}^p(-1)^{n+1}frac{(2n-3)!!}{(2n+3)!!}x^p$$ It write
$$S_p(x)=frac{left(8 p^3+36 p^2+46 p+15right) ,
_2F_1left(-frac{1}{2},1;frac{5}{2};-xright)+3 (-x)^{p+1} ,
_2F_1left(1,p+frac{1}{2};p+frac{7}{2};-xright)}{3 left(8 p^3+36 p^2+46
p+15right)}$$ Using
$$,
_2F_1left(-frac{1}{2},1;frac{5}{2};-1right)=frac{3 pi }{8}$$ then
$$S_p(1)=frac{pi left(8 p^3+36 p^2+46 p+15right)-8 (-1)^p ,
_2F_1left(1,p+frac{1}{2};p+frac{7}{2};-1right)}{8 left(8 p^3+36 p^2+46
p+15right)}$$ that it to say
$$S_p(1)=frac pi 8-frac{(-1)^p }{8 p^3+36
p^2+46 p+15}, _2F_1left(1,p+frac{1}{2};p+frac{7}{2};-1right)$$ and $, _2F_1left(1,p+frac{1}{2};p+frac{7}{2};-1right)$ looks like an hyperbolic function going asymptotically to $frac 12$.
Then $S_infty(1)=frac{ pi }{8}$.
$endgroup$
Considering the simplicity of Micah's answer, I am ashamed to provide this complex one.
Let
$$S_p(x)=sum_{n=0}^p(-1)^{n+1}frac{(2n-3)!!}{(2n+3)!!}x^p$$ It write
$$S_p(x)=frac{left(8 p^3+36 p^2+46 p+15right) ,
_2F_1left(-frac{1}{2},1;frac{5}{2};-xright)+3 (-x)^{p+1} ,
_2F_1left(1,p+frac{1}{2};p+frac{7}{2};-xright)}{3 left(8 p^3+36 p^2+46
p+15right)}$$ Using
$$,
_2F_1left(-frac{1}{2},1;frac{5}{2};-1right)=frac{3 pi }{8}$$ then
$$S_p(1)=frac{pi left(8 p^3+36 p^2+46 p+15right)-8 (-1)^p ,
_2F_1left(1,p+frac{1}{2};p+frac{7}{2};-1right)}{8 left(8 p^3+36 p^2+46
p+15right)}$$ that it to say
$$S_p(1)=frac pi 8-frac{(-1)^p }{8 p^3+36
p^2+46 p+15}, _2F_1left(1,p+frac{1}{2};p+frac{7}{2};-1right)$$ and $, _2F_1left(1,p+frac{1}{2};p+frac{7}{2};-1right)$ looks like an hyperbolic function going asymptotically to $frac 12$.
Then $S_infty(1)=frac{ pi }{8}$.
answered Dec 31 '18 at 4:34
Claude LeiboviciClaude Leibovici
123k1157134
123k1157134
add a comment |
add a comment |
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$begingroup$
For my own amusement, I will choose to believe this user is actually Sarah Silverman, the actress and comedian.
$endgroup$
– The Count
Dec 30 '18 at 23:36