Closed form of this type $sum_{j=0}^{infty}frac{2^jj^n}{(2j+1)(2j+3){2j choose j}}$












5












$begingroup$


Given that, $$sum_{j=0}^{infty}frac{2^jleft(j-frac{1}{3}right)^3left(j^2+j-1right)}{(2j+1)(2j+3){2j choose j}}=Atag1$$



We have $A=2pi+12+frac{1}{3}?$



We can generalize the above $(1)$:



$$sum_{j=0}^{infty}frac{2^jj^n}{(2j+1)(2j+3){2j choose j}}=F(n)tag2$$



I believe the closed for $(2)$ is



$$F(n)=(-1)^{n}(2n+1)-(-1)^n (2n-1)cdot frac{pi}{2}$$



How can we show that the proposed $(2)$ is correct?










share|cite|improve this question











$endgroup$












  • $begingroup$
    No, $(2)$ is not correct.I doubt there's a closed form for the general sum.
    $endgroup$
    – Mariusz Iwaniuk
    Dec 27 '18 at 20:30












  • $begingroup$
    Do you want $j$ starting at 0 or 1?
    $endgroup$
    – JimB
    Dec 28 '18 at 3:13
















5












$begingroup$


Given that, $$sum_{j=0}^{infty}frac{2^jleft(j-frac{1}{3}right)^3left(j^2+j-1right)}{(2j+1)(2j+3){2j choose j}}=Atag1$$



We have $A=2pi+12+frac{1}{3}?$



We can generalize the above $(1)$:



$$sum_{j=0}^{infty}frac{2^jj^n}{(2j+1)(2j+3){2j choose j}}=F(n)tag2$$



I believe the closed for $(2)$ is



$$F(n)=(-1)^{n}(2n+1)-(-1)^n (2n-1)cdot frac{pi}{2}$$



How can we show that the proposed $(2)$ is correct?










share|cite|improve this question











$endgroup$












  • $begingroup$
    No, $(2)$ is not correct.I doubt there's a closed form for the general sum.
    $endgroup$
    – Mariusz Iwaniuk
    Dec 27 '18 at 20:30












  • $begingroup$
    Do you want $j$ starting at 0 or 1?
    $endgroup$
    – JimB
    Dec 28 '18 at 3:13














5












5








5


2



$begingroup$


Given that, $$sum_{j=0}^{infty}frac{2^jleft(j-frac{1}{3}right)^3left(j^2+j-1right)}{(2j+1)(2j+3){2j choose j}}=Atag1$$



We have $A=2pi+12+frac{1}{3}?$



We can generalize the above $(1)$:



$$sum_{j=0}^{infty}frac{2^jj^n}{(2j+1)(2j+3){2j choose j}}=F(n)tag2$$



I believe the closed for $(2)$ is



$$F(n)=(-1)^{n}(2n+1)-(-1)^n (2n-1)cdot frac{pi}{2}$$



How can we show that the proposed $(2)$ is correct?










share|cite|improve this question











$endgroup$




Given that, $$sum_{j=0}^{infty}frac{2^jleft(j-frac{1}{3}right)^3left(j^2+j-1right)}{(2j+1)(2j+3){2j choose j}}=Atag1$$



We have $A=2pi+12+frac{1}{3}?$



We can generalize the above $(1)$:



$$sum_{j=0}^{infty}frac{2^jj^n}{(2j+1)(2j+3){2j choose j}}=F(n)tag2$$



I believe the closed for $(2)$ is



$$F(n)=(-1)^{n}(2n+1)-(-1)^n (2n-1)cdot frac{pi}{2}$$



How can we show that the proposed $(2)$ is correct?







sequences-and-series closed-form






share|cite|improve this question















share|cite|improve this question













share|cite|improve this question




share|cite|improve this question








edited Dec 27 '18 at 19:27







user583851

















asked Dec 27 '18 at 19:15









user583851user583851

1




1












  • $begingroup$
    No, $(2)$ is not correct.I doubt there's a closed form for the general sum.
    $endgroup$
    – Mariusz Iwaniuk
    Dec 27 '18 at 20:30












  • $begingroup$
    Do you want $j$ starting at 0 or 1?
    $endgroup$
    – JimB
    Dec 28 '18 at 3:13


















  • $begingroup$
    No, $(2)$ is not correct.I doubt there's a closed form for the general sum.
    $endgroup$
    – Mariusz Iwaniuk
    Dec 27 '18 at 20:30












  • $begingroup$
    Do you want $j$ starting at 0 or 1?
    $endgroup$
    – JimB
    Dec 28 '18 at 3:13
















$begingroup$
No, $(2)$ is not correct.I doubt there's a closed form for the general sum.
$endgroup$
– Mariusz Iwaniuk
Dec 27 '18 at 20:30






$begingroup$
No, $(2)$ is not correct.I doubt there's a closed form for the general sum.
$endgroup$
– Mariusz Iwaniuk
Dec 27 '18 at 20:30














$begingroup$
Do you want $j$ starting at 0 or 1?
$endgroup$
– JimB
Dec 28 '18 at 3:13




$begingroup$
Do you want $j$ starting at 0 or 1?
$endgroup$
– JimB
Dec 28 '18 at 3:13










2 Answers
2






active

oldest

votes


















9












$begingroup$

From the expansion of $arcsin^2 t$
begin{equation}
arcsin^2 t=sum_{p=0}^infty frac{2^{2p}t^{2p+2}}{(2p+1)(p+1)binom{2p}{p}}
end{equation}

we can obtain by differentiation
begin{equation}
2frac{arcsin t}{sqrt{1-t^2}}=sum_{p=0}^infty frac{2^{2p+1}t^{2p+1}}{(2p+1)binom{2p}{p}}
end{equation}

Multiplying the above identity by $t$ and integrating, one obtains
begin{equation}
2int_0^xfrac{tarcsin t}{sqrt{1-t^2}},dt=sum_{p=0}^infty frac{2^{2p+1}x^{2p+3}}{(2p+1)(2p+3)binom{2p}{p}}
end{equation}

We choose $x=sqrt{y}/2$ and divide the identity by $y^{3/2}$ to write
begin{equation}
frac{8}{y^{3/2}}int_0^{sqrt{y}/2}frac{tarcsin t}{sqrt{1-t^2}},dt=sum_{p=0}^infty frac{y^{p}}{(2p+1)(2p+3)binom{2p}{p}}
end{equation}

Now, applying the operator $yfrac{d}{dy}$ $n$ times and taking the result at $y=2$ gives
begin{equation}
left.left[ yfrac{d}{dy}right]^nfrac{8}{y^{3/2}}int_0^{sqrt{y}/2}frac{tarcsin t}{sqrt{1-t^2}},dtright|_{y=2}=sum_{p=0}^infty frac{p^{n}2^p}{(2p+1)(2p+3)binom{2p}{p}}
end{equation}

The function can be evaluated as
begin{equation}
frac{8}{y^{3/2}}int_0^{sqrt{y}/2}frac{tarcsin t}{sqrt{1-t^2}},dt=frac{4}{y}-8frac{sqrt{1-frac{y}{4}}}{y^{3/2}}arcsinleft( frac{sqrt{y}}{2} right)
end{equation}

The above result can be simplified by taking $z=sqrt{y}/2$:
begin{equation}
left.left[frac{1}{2} zfrac{d}{dz}right]^n
left[ frac{1}{z^2}-frac{sqrt{1-z^2}}{z^3}arcsin zright]
right|_{z=sqrt{2}/2}=sum_{p=0}^infty frac{p^{n}2^p}{(2p+1)(2p+3)binom{2p}{p}}
end{equation}

As $arcsin left(sqrt{2}/2right)=pi/4$ and since the successive applications of the operator on $z^{-2}$ and $z^{-3}left( 1-z^2 right)^k$ ($k$ is an integer) at $z=1/sqrt{2}$ give rational results, we expect
begin{equation}
F(n)=a_n+b_npi
end{equation}

where $a_n$ and $b_n$ are rational. I could not find any simple expression for these coefficients however. First few values are $F(n)=2-pi/2,-3+pi,5-3pi/2,-7+5pi/2,13-3pi,-7+17pi/2, 93+27pi/2cdots$ for $n=0,1,2,3,4,5,6cdots$. They do not correspond to the propose formula as remarked by @Mariusz Iwaniuk in a comment.



With $z=exp(t/2)$, we have $1/2zd/dz=d/dt$, and we can build the generating function for the $F(n)$:
begin{equation}
sum_{n=0}^infty F(n)frac{t^n}{n!}=phileft( t-ln2 right)
end{equation}

where
begin{equation}
phileft( t-ln 2 right)=2e^{-3t/2}left[ e^{t/2}-sqrt{2-e^{t}}arcsinleft(frac{ e^{t/2}}{sqrt{2}} right)right]
end{equation}






share|cite|improve this answer











$endgroup$













  • $begingroup$
    (+1) Maybe I'm doing something wrong but I get (what I think are the correct results) by removing the multiplier of 2 from your generating function but getting $F(n)-(-1)^n$ from the generating function. That makes the first 7 values $2-frac{pi }{2},pi -3,5-frac{3 pi }{2},frac{5 pi }{2}-7,13-3 pi ,frac{17 pi }{2}-7,93+frac{27 pi }{2}$. Am I totally screwing up here?
    $endgroup$
    – JimB
    Dec 28 '18 at 2:38












  • $begingroup$
    @JimB You're right. Thank you for having pointed the typo which is now corrected.
    $endgroup$
    – Paul Enta
    Dec 28 '18 at 10:22












  • $begingroup$
    In yours formula in Mathematica for n=2: Expand[(z/2*D[1/z^2 - Sqrt[1 - z^2]/z^3*ArcSin[z], {z, 2}]) /. z -> Sqrt[2]/2] I have got: 13 Sqrt[2] - 4 Sqrt[2] π ,but it should be: 5 - (3 π)/2 ?
    $endgroup$
    – Mariusz Iwaniuk
    Dec 28 '18 at 13:17












  • $begingroup$
    @MariuszIwaniuk I think that for $n=2$ the expression reads $$frac z2 frac{d}{dz}left[frac z2 frac{d}{dz} f(z)right]$$ and not $frac z2frac{d^2}{dz^2} f(z)$.
    $endgroup$
    – Paul Enta
    Dec 28 '18 at 14:14












  • $begingroup$
    @PaulEnta It works.Thanks.
    $endgroup$
    – Mariusz Iwaniuk
    Dec 28 '18 at 14:36



















3












$begingroup$

Too long for a comment.



Just out of curiosity, I computed $$G_n=2(-1)^nsum_{j=0}^{infty}frac{2^jj^n}{(2j+1)(2j+3){2j choose j}}$$ and got the surprising results
$$left(
begin{array}{cc}
0 & 4-pi \
1 & 6-2 pi \
2 & 10-3 pi \
3 & 14-5 pi \
4 & 26-6 pi \
5 & 14-17 pi \
6 & 186+27 pi \
7 & -1042-380 pi \
8 & 10906+3399 pi \
9 & -119218-38057 pi \
10 & 1490042+474132 pi \
11 & -20706898-6591455 pi \
12 & 317327706+101008179 pi \
13 & -5315948530-1692119522 pi \
14 & 96656651578+30766766937 pi \
15 & -1895852523154-603468602105 pi \
16 & 39903246182426+12701597748834 pi \
17 & -897111926876722-285559595341037 pi \
18 & 21456828988070394+6829920793053567 pi \
19 & -544015833214835410-173165617952800580 pi \
20 & 14574701046924559066+4639271431409321739 pi
end{array}
right)$$
where, as already said in comments and answers, no pattern seems to appear.






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    2 Answers
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    2 Answers
    2






    active

    oldest

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    active

    oldest

    votes






    active

    oldest

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    9












    $begingroup$

    From the expansion of $arcsin^2 t$
    begin{equation}
    arcsin^2 t=sum_{p=0}^infty frac{2^{2p}t^{2p+2}}{(2p+1)(p+1)binom{2p}{p}}
    end{equation}

    we can obtain by differentiation
    begin{equation}
    2frac{arcsin t}{sqrt{1-t^2}}=sum_{p=0}^infty frac{2^{2p+1}t^{2p+1}}{(2p+1)binom{2p}{p}}
    end{equation}

    Multiplying the above identity by $t$ and integrating, one obtains
    begin{equation}
    2int_0^xfrac{tarcsin t}{sqrt{1-t^2}},dt=sum_{p=0}^infty frac{2^{2p+1}x^{2p+3}}{(2p+1)(2p+3)binom{2p}{p}}
    end{equation}

    We choose $x=sqrt{y}/2$ and divide the identity by $y^{3/2}$ to write
    begin{equation}
    frac{8}{y^{3/2}}int_0^{sqrt{y}/2}frac{tarcsin t}{sqrt{1-t^2}},dt=sum_{p=0}^infty frac{y^{p}}{(2p+1)(2p+3)binom{2p}{p}}
    end{equation}

    Now, applying the operator $yfrac{d}{dy}$ $n$ times and taking the result at $y=2$ gives
    begin{equation}
    left.left[ yfrac{d}{dy}right]^nfrac{8}{y^{3/2}}int_0^{sqrt{y}/2}frac{tarcsin t}{sqrt{1-t^2}},dtright|_{y=2}=sum_{p=0}^infty frac{p^{n}2^p}{(2p+1)(2p+3)binom{2p}{p}}
    end{equation}

    The function can be evaluated as
    begin{equation}
    frac{8}{y^{3/2}}int_0^{sqrt{y}/2}frac{tarcsin t}{sqrt{1-t^2}},dt=frac{4}{y}-8frac{sqrt{1-frac{y}{4}}}{y^{3/2}}arcsinleft( frac{sqrt{y}}{2} right)
    end{equation}

    The above result can be simplified by taking $z=sqrt{y}/2$:
    begin{equation}
    left.left[frac{1}{2} zfrac{d}{dz}right]^n
    left[ frac{1}{z^2}-frac{sqrt{1-z^2}}{z^3}arcsin zright]
    right|_{z=sqrt{2}/2}=sum_{p=0}^infty frac{p^{n}2^p}{(2p+1)(2p+3)binom{2p}{p}}
    end{equation}

    As $arcsin left(sqrt{2}/2right)=pi/4$ and since the successive applications of the operator on $z^{-2}$ and $z^{-3}left( 1-z^2 right)^k$ ($k$ is an integer) at $z=1/sqrt{2}$ give rational results, we expect
    begin{equation}
    F(n)=a_n+b_npi
    end{equation}

    where $a_n$ and $b_n$ are rational. I could not find any simple expression for these coefficients however. First few values are $F(n)=2-pi/2,-3+pi,5-3pi/2,-7+5pi/2,13-3pi,-7+17pi/2, 93+27pi/2cdots$ for $n=0,1,2,3,4,5,6cdots$. They do not correspond to the propose formula as remarked by @Mariusz Iwaniuk in a comment.



    With $z=exp(t/2)$, we have $1/2zd/dz=d/dt$, and we can build the generating function for the $F(n)$:
    begin{equation}
    sum_{n=0}^infty F(n)frac{t^n}{n!}=phileft( t-ln2 right)
    end{equation}

    where
    begin{equation}
    phileft( t-ln 2 right)=2e^{-3t/2}left[ e^{t/2}-sqrt{2-e^{t}}arcsinleft(frac{ e^{t/2}}{sqrt{2}} right)right]
    end{equation}






    share|cite|improve this answer











    $endgroup$













    • $begingroup$
      (+1) Maybe I'm doing something wrong but I get (what I think are the correct results) by removing the multiplier of 2 from your generating function but getting $F(n)-(-1)^n$ from the generating function. That makes the first 7 values $2-frac{pi }{2},pi -3,5-frac{3 pi }{2},frac{5 pi }{2}-7,13-3 pi ,frac{17 pi }{2}-7,93+frac{27 pi }{2}$. Am I totally screwing up here?
      $endgroup$
      – JimB
      Dec 28 '18 at 2:38












    • $begingroup$
      @JimB You're right. Thank you for having pointed the typo which is now corrected.
      $endgroup$
      – Paul Enta
      Dec 28 '18 at 10:22












    • $begingroup$
      In yours formula in Mathematica for n=2: Expand[(z/2*D[1/z^2 - Sqrt[1 - z^2]/z^3*ArcSin[z], {z, 2}]) /. z -> Sqrt[2]/2] I have got: 13 Sqrt[2] - 4 Sqrt[2] π ,but it should be: 5 - (3 π)/2 ?
      $endgroup$
      – Mariusz Iwaniuk
      Dec 28 '18 at 13:17












    • $begingroup$
      @MariuszIwaniuk I think that for $n=2$ the expression reads $$frac z2 frac{d}{dz}left[frac z2 frac{d}{dz} f(z)right]$$ and not $frac z2frac{d^2}{dz^2} f(z)$.
      $endgroup$
      – Paul Enta
      Dec 28 '18 at 14:14












    • $begingroup$
      @PaulEnta It works.Thanks.
      $endgroup$
      – Mariusz Iwaniuk
      Dec 28 '18 at 14:36
















    9












    $begingroup$

    From the expansion of $arcsin^2 t$
    begin{equation}
    arcsin^2 t=sum_{p=0}^infty frac{2^{2p}t^{2p+2}}{(2p+1)(p+1)binom{2p}{p}}
    end{equation}

    we can obtain by differentiation
    begin{equation}
    2frac{arcsin t}{sqrt{1-t^2}}=sum_{p=0}^infty frac{2^{2p+1}t^{2p+1}}{(2p+1)binom{2p}{p}}
    end{equation}

    Multiplying the above identity by $t$ and integrating, one obtains
    begin{equation}
    2int_0^xfrac{tarcsin t}{sqrt{1-t^2}},dt=sum_{p=0}^infty frac{2^{2p+1}x^{2p+3}}{(2p+1)(2p+3)binom{2p}{p}}
    end{equation}

    We choose $x=sqrt{y}/2$ and divide the identity by $y^{3/2}$ to write
    begin{equation}
    frac{8}{y^{3/2}}int_0^{sqrt{y}/2}frac{tarcsin t}{sqrt{1-t^2}},dt=sum_{p=0}^infty frac{y^{p}}{(2p+1)(2p+3)binom{2p}{p}}
    end{equation}

    Now, applying the operator $yfrac{d}{dy}$ $n$ times and taking the result at $y=2$ gives
    begin{equation}
    left.left[ yfrac{d}{dy}right]^nfrac{8}{y^{3/2}}int_0^{sqrt{y}/2}frac{tarcsin t}{sqrt{1-t^2}},dtright|_{y=2}=sum_{p=0}^infty frac{p^{n}2^p}{(2p+1)(2p+3)binom{2p}{p}}
    end{equation}

    The function can be evaluated as
    begin{equation}
    frac{8}{y^{3/2}}int_0^{sqrt{y}/2}frac{tarcsin t}{sqrt{1-t^2}},dt=frac{4}{y}-8frac{sqrt{1-frac{y}{4}}}{y^{3/2}}arcsinleft( frac{sqrt{y}}{2} right)
    end{equation}

    The above result can be simplified by taking $z=sqrt{y}/2$:
    begin{equation}
    left.left[frac{1}{2} zfrac{d}{dz}right]^n
    left[ frac{1}{z^2}-frac{sqrt{1-z^2}}{z^3}arcsin zright]
    right|_{z=sqrt{2}/2}=sum_{p=0}^infty frac{p^{n}2^p}{(2p+1)(2p+3)binom{2p}{p}}
    end{equation}

    As $arcsin left(sqrt{2}/2right)=pi/4$ and since the successive applications of the operator on $z^{-2}$ and $z^{-3}left( 1-z^2 right)^k$ ($k$ is an integer) at $z=1/sqrt{2}$ give rational results, we expect
    begin{equation}
    F(n)=a_n+b_npi
    end{equation}

    where $a_n$ and $b_n$ are rational. I could not find any simple expression for these coefficients however. First few values are $F(n)=2-pi/2,-3+pi,5-3pi/2,-7+5pi/2,13-3pi,-7+17pi/2, 93+27pi/2cdots$ for $n=0,1,2,3,4,5,6cdots$. They do not correspond to the propose formula as remarked by @Mariusz Iwaniuk in a comment.



    With $z=exp(t/2)$, we have $1/2zd/dz=d/dt$, and we can build the generating function for the $F(n)$:
    begin{equation}
    sum_{n=0}^infty F(n)frac{t^n}{n!}=phileft( t-ln2 right)
    end{equation}

    where
    begin{equation}
    phileft( t-ln 2 right)=2e^{-3t/2}left[ e^{t/2}-sqrt{2-e^{t}}arcsinleft(frac{ e^{t/2}}{sqrt{2}} right)right]
    end{equation}






    share|cite|improve this answer











    $endgroup$













    • $begingroup$
      (+1) Maybe I'm doing something wrong but I get (what I think are the correct results) by removing the multiplier of 2 from your generating function but getting $F(n)-(-1)^n$ from the generating function. That makes the first 7 values $2-frac{pi }{2},pi -3,5-frac{3 pi }{2},frac{5 pi }{2}-7,13-3 pi ,frac{17 pi }{2}-7,93+frac{27 pi }{2}$. Am I totally screwing up here?
      $endgroup$
      – JimB
      Dec 28 '18 at 2:38












    • $begingroup$
      @JimB You're right. Thank you for having pointed the typo which is now corrected.
      $endgroup$
      – Paul Enta
      Dec 28 '18 at 10:22












    • $begingroup$
      In yours formula in Mathematica for n=2: Expand[(z/2*D[1/z^2 - Sqrt[1 - z^2]/z^3*ArcSin[z], {z, 2}]) /. z -> Sqrt[2]/2] I have got: 13 Sqrt[2] - 4 Sqrt[2] π ,but it should be: 5 - (3 π)/2 ?
      $endgroup$
      – Mariusz Iwaniuk
      Dec 28 '18 at 13:17












    • $begingroup$
      @MariuszIwaniuk I think that for $n=2$ the expression reads $$frac z2 frac{d}{dz}left[frac z2 frac{d}{dz} f(z)right]$$ and not $frac z2frac{d^2}{dz^2} f(z)$.
      $endgroup$
      – Paul Enta
      Dec 28 '18 at 14:14












    • $begingroup$
      @PaulEnta It works.Thanks.
      $endgroup$
      – Mariusz Iwaniuk
      Dec 28 '18 at 14:36














    9












    9








    9





    $begingroup$

    From the expansion of $arcsin^2 t$
    begin{equation}
    arcsin^2 t=sum_{p=0}^infty frac{2^{2p}t^{2p+2}}{(2p+1)(p+1)binom{2p}{p}}
    end{equation}

    we can obtain by differentiation
    begin{equation}
    2frac{arcsin t}{sqrt{1-t^2}}=sum_{p=0}^infty frac{2^{2p+1}t^{2p+1}}{(2p+1)binom{2p}{p}}
    end{equation}

    Multiplying the above identity by $t$ and integrating, one obtains
    begin{equation}
    2int_0^xfrac{tarcsin t}{sqrt{1-t^2}},dt=sum_{p=0}^infty frac{2^{2p+1}x^{2p+3}}{(2p+1)(2p+3)binom{2p}{p}}
    end{equation}

    We choose $x=sqrt{y}/2$ and divide the identity by $y^{3/2}$ to write
    begin{equation}
    frac{8}{y^{3/2}}int_0^{sqrt{y}/2}frac{tarcsin t}{sqrt{1-t^2}},dt=sum_{p=0}^infty frac{y^{p}}{(2p+1)(2p+3)binom{2p}{p}}
    end{equation}

    Now, applying the operator $yfrac{d}{dy}$ $n$ times and taking the result at $y=2$ gives
    begin{equation}
    left.left[ yfrac{d}{dy}right]^nfrac{8}{y^{3/2}}int_0^{sqrt{y}/2}frac{tarcsin t}{sqrt{1-t^2}},dtright|_{y=2}=sum_{p=0}^infty frac{p^{n}2^p}{(2p+1)(2p+3)binom{2p}{p}}
    end{equation}

    The function can be evaluated as
    begin{equation}
    frac{8}{y^{3/2}}int_0^{sqrt{y}/2}frac{tarcsin t}{sqrt{1-t^2}},dt=frac{4}{y}-8frac{sqrt{1-frac{y}{4}}}{y^{3/2}}arcsinleft( frac{sqrt{y}}{2} right)
    end{equation}

    The above result can be simplified by taking $z=sqrt{y}/2$:
    begin{equation}
    left.left[frac{1}{2} zfrac{d}{dz}right]^n
    left[ frac{1}{z^2}-frac{sqrt{1-z^2}}{z^3}arcsin zright]
    right|_{z=sqrt{2}/2}=sum_{p=0}^infty frac{p^{n}2^p}{(2p+1)(2p+3)binom{2p}{p}}
    end{equation}

    As $arcsin left(sqrt{2}/2right)=pi/4$ and since the successive applications of the operator on $z^{-2}$ and $z^{-3}left( 1-z^2 right)^k$ ($k$ is an integer) at $z=1/sqrt{2}$ give rational results, we expect
    begin{equation}
    F(n)=a_n+b_npi
    end{equation}

    where $a_n$ and $b_n$ are rational. I could not find any simple expression for these coefficients however. First few values are $F(n)=2-pi/2,-3+pi,5-3pi/2,-7+5pi/2,13-3pi,-7+17pi/2, 93+27pi/2cdots$ for $n=0,1,2,3,4,5,6cdots$. They do not correspond to the propose formula as remarked by @Mariusz Iwaniuk in a comment.



    With $z=exp(t/2)$, we have $1/2zd/dz=d/dt$, and we can build the generating function for the $F(n)$:
    begin{equation}
    sum_{n=0}^infty F(n)frac{t^n}{n!}=phileft( t-ln2 right)
    end{equation}

    where
    begin{equation}
    phileft( t-ln 2 right)=2e^{-3t/2}left[ e^{t/2}-sqrt{2-e^{t}}arcsinleft(frac{ e^{t/2}}{sqrt{2}} right)right]
    end{equation}






    share|cite|improve this answer











    $endgroup$



    From the expansion of $arcsin^2 t$
    begin{equation}
    arcsin^2 t=sum_{p=0}^infty frac{2^{2p}t^{2p+2}}{(2p+1)(p+1)binom{2p}{p}}
    end{equation}

    we can obtain by differentiation
    begin{equation}
    2frac{arcsin t}{sqrt{1-t^2}}=sum_{p=0}^infty frac{2^{2p+1}t^{2p+1}}{(2p+1)binom{2p}{p}}
    end{equation}

    Multiplying the above identity by $t$ and integrating, one obtains
    begin{equation}
    2int_0^xfrac{tarcsin t}{sqrt{1-t^2}},dt=sum_{p=0}^infty frac{2^{2p+1}x^{2p+3}}{(2p+1)(2p+3)binom{2p}{p}}
    end{equation}

    We choose $x=sqrt{y}/2$ and divide the identity by $y^{3/2}$ to write
    begin{equation}
    frac{8}{y^{3/2}}int_0^{sqrt{y}/2}frac{tarcsin t}{sqrt{1-t^2}},dt=sum_{p=0}^infty frac{y^{p}}{(2p+1)(2p+3)binom{2p}{p}}
    end{equation}

    Now, applying the operator $yfrac{d}{dy}$ $n$ times and taking the result at $y=2$ gives
    begin{equation}
    left.left[ yfrac{d}{dy}right]^nfrac{8}{y^{3/2}}int_0^{sqrt{y}/2}frac{tarcsin t}{sqrt{1-t^2}},dtright|_{y=2}=sum_{p=0}^infty frac{p^{n}2^p}{(2p+1)(2p+3)binom{2p}{p}}
    end{equation}

    The function can be evaluated as
    begin{equation}
    frac{8}{y^{3/2}}int_0^{sqrt{y}/2}frac{tarcsin t}{sqrt{1-t^2}},dt=frac{4}{y}-8frac{sqrt{1-frac{y}{4}}}{y^{3/2}}arcsinleft( frac{sqrt{y}}{2} right)
    end{equation}

    The above result can be simplified by taking $z=sqrt{y}/2$:
    begin{equation}
    left.left[frac{1}{2} zfrac{d}{dz}right]^n
    left[ frac{1}{z^2}-frac{sqrt{1-z^2}}{z^3}arcsin zright]
    right|_{z=sqrt{2}/2}=sum_{p=0}^infty frac{p^{n}2^p}{(2p+1)(2p+3)binom{2p}{p}}
    end{equation}

    As $arcsin left(sqrt{2}/2right)=pi/4$ and since the successive applications of the operator on $z^{-2}$ and $z^{-3}left( 1-z^2 right)^k$ ($k$ is an integer) at $z=1/sqrt{2}$ give rational results, we expect
    begin{equation}
    F(n)=a_n+b_npi
    end{equation}

    where $a_n$ and $b_n$ are rational. I could not find any simple expression for these coefficients however. First few values are $F(n)=2-pi/2,-3+pi,5-3pi/2,-7+5pi/2,13-3pi,-7+17pi/2, 93+27pi/2cdots$ for $n=0,1,2,3,4,5,6cdots$. They do not correspond to the propose formula as remarked by @Mariusz Iwaniuk in a comment.



    With $z=exp(t/2)$, we have $1/2zd/dz=d/dt$, and we can build the generating function for the $F(n)$:
    begin{equation}
    sum_{n=0}^infty F(n)frac{t^n}{n!}=phileft( t-ln2 right)
    end{equation}

    where
    begin{equation}
    phileft( t-ln 2 right)=2e^{-3t/2}left[ e^{t/2}-sqrt{2-e^{t}}arcsinleft(frac{ e^{t/2}}{sqrt{2}} right)right]
    end{equation}







    share|cite|improve this answer














    share|cite|improve this answer



    share|cite|improve this answer








    edited Dec 28 '18 at 10:22

























    answered Dec 27 '18 at 22:16









    Paul EntaPaul Enta

    5,15111334




    5,15111334












    • $begingroup$
      (+1) Maybe I'm doing something wrong but I get (what I think are the correct results) by removing the multiplier of 2 from your generating function but getting $F(n)-(-1)^n$ from the generating function. That makes the first 7 values $2-frac{pi }{2},pi -3,5-frac{3 pi }{2},frac{5 pi }{2}-7,13-3 pi ,frac{17 pi }{2}-7,93+frac{27 pi }{2}$. Am I totally screwing up here?
      $endgroup$
      – JimB
      Dec 28 '18 at 2:38












    • $begingroup$
      @JimB You're right. Thank you for having pointed the typo which is now corrected.
      $endgroup$
      – Paul Enta
      Dec 28 '18 at 10:22












    • $begingroup$
      In yours formula in Mathematica for n=2: Expand[(z/2*D[1/z^2 - Sqrt[1 - z^2]/z^3*ArcSin[z], {z, 2}]) /. z -> Sqrt[2]/2] I have got: 13 Sqrt[2] - 4 Sqrt[2] π ,but it should be: 5 - (3 π)/2 ?
      $endgroup$
      – Mariusz Iwaniuk
      Dec 28 '18 at 13:17












    • $begingroup$
      @MariuszIwaniuk I think that for $n=2$ the expression reads $$frac z2 frac{d}{dz}left[frac z2 frac{d}{dz} f(z)right]$$ and not $frac z2frac{d^2}{dz^2} f(z)$.
      $endgroup$
      – Paul Enta
      Dec 28 '18 at 14:14












    • $begingroup$
      @PaulEnta It works.Thanks.
      $endgroup$
      – Mariusz Iwaniuk
      Dec 28 '18 at 14:36


















    • $begingroup$
      (+1) Maybe I'm doing something wrong but I get (what I think are the correct results) by removing the multiplier of 2 from your generating function but getting $F(n)-(-1)^n$ from the generating function. That makes the first 7 values $2-frac{pi }{2},pi -3,5-frac{3 pi }{2},frac{5 pi }{2}-7,13-3 pi ,frac{17 pi }{2}-7,93+frac{27 pi }{2}$. Am I totally screwing up here?
      $endgroup$
      – JimB
      Dec 28 '18 at 2:38












    • $begingroup$
      @JimB You're right. Thank you for having pointed the typo which is now corrected.
      $endgroup$
      – Paul Enta
      Dec 28 '18 at 10:22












    • $begingroup$
      In yours formula in Mathematica for n=2: Expand[(z/2*D[1/z^2 - Sqrt[1 - z^2]/z^3*ArcSin[z], {z, 2}]) /. z -> Sqrt[2]/2] I have got: 13 Sqrt[2] - 4 Sqrt[2] π ,but it should be: 5 - (3 π)/2 ?
      $endgroup$
      – Mariusz Iwaniuk
      Dec 28 '18 at 13:17












    • $begingroup$
      @MariuszIwaniuk I think that for $n=2$ the expression reads $$frac z2 frac{d}{dz}left[frac z2 frac{d}{dz} f(z)right]$$ and not $frac z2frac{d^2}{dz^2} f(z)$.
      $endgroup$
      – Paul Enta
      Dec 28 '18 at 14:14












    • $begingroup$
      @PaulEnta It works.Thanks.
      $endgroup$
      – Mariusz Iwaniuk
      Dec 28 '18 at 14:36
















    $begingroup$
    (+1) Maybe I'm doing something wrong but I get (what I think are the correct results) by removing the multiplier of 2 from your generating function but getting $F(n)-(-1)^n$ from the generating function. That makes the first 7 values $2-frac{pi }{2},pi -3,5-frac{3 pi }{2},frac{5 pi }{2}-7,13-3 pi ,frac{17 pi }{2}-7,93+frac{27 pi }{2}$. Am I totally screwing up here?
    $endgroup$
    – JimB
    Dec 28 '18 at 2:38






    $begingroup$
    (+1) Maybe I'm doing something wrong but I get (what I think are the correct results) by removing the multiplier of 2 from your generating function but getting $F(n)-(-1)^n$ from the generating function. That makes the first 7 values $2-frac{pi }{2},pi -3,5-frac{3 pi }{2},frac{5 pi }{2}-7,13-3 pi ,frac{17 pi }{2}-7,93+frac{27 pi }{2}$. Am I totally screwing up here?
    $endgroup$
    – JimB
    Dec 28 '18 at 2:38














    $begingroup$
    @JimB You're right. Thank you for having pointed the typo which is now corrected.
    $endgroup$
    – Paul Enta
    Dec 28 '18 at 10:22






    $begingroup$
    @JimB You're right. Thank you for having pointed the typo which is now corrected.
    $endgroup$
    – Paul Enta
    Dec 28 '18 at 10:22














    $begingroup$
    In yours formula in Mathematica for n=2: Expand[(z/2*D[1/z^2 - Sqrt[1 - z^2]/z^3*ArcSin[z], {z, 2}]) /. z -> Sqrt[2]/2] I have got: 13 Sqrt[2] - 4 Sqrt[2] π ,but it should be: 5 - (3 π)/2 ?
    $endgroup$
    – Mariusz Iwaniuk
    Dec 28 '18 at 13:17






    $begingroup$
    In yours formula in Mathematica for n=2: Expand[(z/2*D[1/z^2 - Sqrt[1 - z^2]/z^3*ArcSin[z], {z, 2}]) /. z -> Sqrt[2]/2] I have got: 13 Sqrt[2] - 4 Sqrt[2] π ,but it should be: 5 - (3 π)/2 ?
    $endgroup$
    – Mariusz Iwaniuk
    Dec 28 '18 at 13:17














    $begingroup$
    @MariuszIwaniuk I think that for $n=2$ the expression reads $$frac z2 frac{d}{dz}left[frac z2 frac{d}{dz} f(z)right]$$ and not $frac z2frac{d^2}{dz^2} f(z)$.
    $endgroup$
    – Paul Enta
    Dec 28 '18 at 14:14






    $begingroup$
    @MariuszIwaniuk I think that for $n=2$ the expression reads $$frac z2 frac{d}{dz}left[frac z2 frac{d}{dz} f(z)right]$$ and not $frac z2frac{d^2}{dz^2} f(z)$.
    $endgroup$
    – Paul Enta
    Dec 28 '18 at 14:14














    $begingroup$
    @PaulEnta It works.Thanks.
    $endgroup$
    – Mariusz Iwaniuk
    Dec 28 '18 at 14:36




    $begingroup$
    @PaulEnta It works.Thanks.
    $endgroup$
    – Mariusz Iwaniuk
    Dec 28 '18 at 14:36











    3












    $begingroup$

    Too long for a comment.



    Just out of curiosity, I computed $$G_n=2(-1)^nsum_{j=0}^{infty}frac{2^jj^n}{(2j+1)(2j+3){2j choose j}}$$ and got the surprising results
    $$left(
    begin{array}{cc}
    0 & 4-pi \
    1 & 6-2 pi \
    2 & 10-3 pi \
    3 & 14-5 pi \
    4 & 26-6 pi \
    5 & 14-17 pi \
    6 & 186+27 pi \
    7 & -1042-380 pi \
    8 & 10906+3399 pi \
    9 & -119218-38057 pi \
    10 & 1490042+474132 pi \
    11 & -20706898-6591455 pi \
    12 & 317327706+101008179 pi \
    13 & -5315948530-1692119522 pi \
    14 & 96656651578+30766766937 pi \
    15 & -1895852523154-603468602105 pi \
    16 & 39903246182426+12701597748834 pi \
    17 & -897111926876722-285559595341037 pi \
    18 & 21456828988070394+6829920793053567 pi \
    19 & -544015833214835410-173165617952800580 pi \
    20 & 14574701046924559066+4639271431409321739 pi
    end{array}
    right)$$
    where, as already said in comments and answers, no pattern seems to appear.






    share|cite|improve this answer









    $endgroup$


















      3












      $begingroup$

      Too long for a comment.



      Just out of curiosity, I computed $$G_n=2(-1)^nsum_{j=0}^{infty}frac{2^jj^n}{(2j+1)(2j+3){2j choose j}}$$ and got the surprising results
      $$left(
      begin{array}{cc}
      0 & 4-pi \
      1 & 6-2 pi \
      2 & 10-3 pi \
      3 & 14-5 pi \
      4 & 26-6 pi \
      5 & 14-17 pi \
      6 & 186+27 pi \
      7 & -1042-380 pi \
      8 & 10906+3399 pi \
      9 & -119218-38057 pi \
      10 & 1490042+474132 pi \
      11 & -20706898-6591455 pi \
      12 & 317327706+101008179 pi \
      13 & -5315948530-1692119522 pi \
      14 & 96656651578+30766766937 pi \
      15 & -1895852523154-603468602105 pi \
      16 & 39903246182426+12701597748834 pi \
      17 & -897111926876722-285559595341037 pi \
      18 & 21456828988070394+6829920793053567 pi \
      19 & -544015833214835410-173165617952800580 pi \
      20 & 14574701046924559066+4639271431409321739 pi
      end{array}
      right)$$
      where, as already said in comments and answers, no pattern seems to appear.






      share|cite|improve this answer









      $endgroup$
















        3












        3








        3





        $begingroup$

        Too long for a comment.



        Just out of curiosity, I computed $$G_n=2(-1)^nsum_{j=0}^{infty}frac{2^jj^n}{(2j+1)(2j+3){2j choose j}}$$ and got the surprising results
        $$left(
        begin{array}{cc}
        0 & 4-pi \
        1 & 6-2 pi \
        2 & 10-3 pi \
        3 & 14-5 pi \
        4 & 26-6 pi \
        5 & 14-17 pi \
        6 & 186+27 pi \
        7 & -1042-380 pi \
        8 & 10906+3399 pi \
        9 & -119218-38057 pi \
        10 & 1490042+474132 pi \
        11 & -20706898-6591455 pi \
        12 & 317327706+101008179 pi \
        13 & -5315948530-1692119522 pi \
        14 & 96656651578+30766766937 pi \
        15 & -1895852523154-603468602105 pi \
        16 & 39903246182426+12701597748834 pi \
        17 & -897111926876722-285559595341037 pi \
        18 & 21456828988070394+6829920793053567 pi \
        19 & -544015833214835410-173165617952800580 pi \
        20 & 14574701046924559066+4639271431409321739 pi
        end{array}
        right)$$
        where, as already said in comments and answers, no pattern seems to appear.






        share|cite|improve this answer









        $endgroup$



        Too long for a comment.



        Just out of curiosity, I computed $$G_n=2(-1)^nsum_{j=0}^{infty}frac{2^jj^n}{(2j+1)(2j+3){2j choose j}}$$ and got the surprising results
        $$left(
        begin{array}{cc}
        0 & 4-pi \
        1 & 6-2 pi \
        2 & 10-3 pi \
        3 & 14-5 pi \
        4 & 26-6 pi \
        5 & 14-17 pi \
        6 & 186+27 pi \
        7 & -1042-380 pi \
        8 & 10906+3399 pi \
        9 & -119218-38057 pi \
        10 & 1490042+474132 pi \
        11 & -20706898-6591455 pi \
        12 & 317327706+101008179 pi \
        13 & -5315948530-1692119522 pi \
        14 & 96656651578+30766766937 pi \
        15 & -1895852523154-603468602105 pi \
        16 & 39903246182426+12701597748834 pi \
        17 & -897111926876722-285559595341037 pi \
        18 & 21456828988070394+6829920793053567 pi \
        19 & -544015833214835410-173165617952800580 pi \
        20 & 14574701046924559066+4639271431409321739 pi
        end{array}
        right)$$
        where, as already said in comments and answers, no pattern seems to appear.







        share|cite|improve this answer












        share|cite|improve this answer



        share|cite|improve this answer










        answered Dec 28 '18 at 5:25









        Claude LeiboviciClaude Leibovici

        122k1157134




        122k1157134






























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