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I would like to calculate the integral $$\int_0^{2\pi} dx \sin^6\left(\frac{x}{2}\right) F\left(\frac{4-n}{2}, \frac{4+n}{2}, \frac{1}{2}, \cos^2 \frac{x}{2} \right)^2$$ where $F$ is the hypergeometric function (Hypergeometric2F1 in Mathematica). One can see in Mathematica that this integral has some well-defined analytic solution when $n$ is an even integer, excluding $0, -2, 2$ (this is related to the fact that the hypergeometric function is a simple polynomial in these cases). For example, $n=4$ gives $5\pi/8$ and $n=6$ gives $45\pi/32$. I would like to obtain an analytic expression for this integral - however, Mathematica obviously cannot solve this for general $n$. I've tried using Assumptions in the integral, but Mathematica still cannot solve the expression for variable $n$.

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    $\begingroup$ Try replacing $n$ with $2 m$ where $m$ is any integer. $\endgroup$
    – flinty
    Commented Jun 3, 2020 at 19:33
  • $\begingroup$ This doesn't work. I don't think the issue is with $n$ being an even integer, but rather the integer assumption more generally - my guess is that Mathematica doesn't try to simplify the hypergeometric function when it assumes $m$ is an integer. $\endgroup$ Commented Jun 3, 2020 at 19:46
  • $\begingroup$ Yes, I just tried it, also tweaking with Refine. Disappointing it cannot simplify. $\endgroup$
    – flinty
    Commented Jun 3, 2020 at 19:48

1 Answer 1

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A = Table[{n, 
      Integrate[Sin[x/2]^6 Hypergeometric2F1[(4-n)/2, (4+n)/2, 1/2, Cos[x/2]^2]^2,
                {x, 0, 2π}]}, {n, 4, 20, 2}]

(*    {{4, 5π/8}, {6, 45π/32}, {8, 33π/16}, {10, 429π/160},
       {12, 1287π/392}, {14, 12155π/3136}, {16, 20995π/4704},
       {18, 323π/64}, {20, 7429π/1320}}                          *)

FindSequenceFunction[A, n] // FullSimplify

(*    (2 (9 - 10 n^2 + n^4) π)/(7 n (-4 + n^2))    *)
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  • $\begingroup$ Wow, I wasn't aware of this function. Thanks! $\endgroup$ Commented Jun 3, 2020 at 19:45

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