Integrate[1/Sqrt[ X^3+1] , x]
This always gives a Hypergeometric solution.
Can it be forced to calculate an Elliptic Integral solution?
Thanks,
Richard
Integrate[1/Sqrt[ X^3+1] , x]
This always gives a Hypergeometric solution.
Can it be forced to calculate an Elliptic Integral solution?
Thanks,
Richard
Using Rubi ( see Rubi - The Rule-based Integrator for Mathematica ), one obtains
Needs@"Rubi`"
int2 = Int[1/Sqrt[x^3 + 1], x]
(*
(2 Sqrt[2 + Sqrt[3]] (1 + x) Sqrt[(1 - x + x^2)/(1 + Sqrt[3] + x)^2]
EllipticF[
ArcSin[(1 - Sqrt[3] + x)/(1 + Sqrt[3] + x)], -7 - 4 Sqrt[3]])/(3^(
1/4) Sqrt[(1 + x)/(1 + Sqrt[3] + x)^2] Sqrt[1 + x^3])
*)
For x > -1, the hypergeometric solution is equal to int2 - (int2 /. x -> 0). However, the hypergeometric solution has simpler branch cuts and differs from this (by a locally constant function) on the whole complex plane. Consequently, the two solutions cannot be simplified into each other. The elliptic function solution can be simplified with the assumption x > -1:
FullSimplify[int2, x > -1]
(*
2 Sqrt[1 + 2/Sqrt[3]]
EllipticF[ArcSin[1 - (2 Sqrt[3])/(1 + Sqrt[3] + x)], -7 - 4 Sqrt[3]]
*)
The hypergeometric solution cannot be simplified into this form under the assumption, since, as an expression tree, it is simpler than the elliptic function expression. I don’t know how to get an elliptic function solution using just the built-in functionality.
Just as a curios observation, we could make integral “close” to elliptic with some Möbius transformation.
I’ve just played with it, like this:
Clear[u, t, x]; u = t/(1 + t);
FullSimplify[
Integrate[FullSimplify[D[u, t]/Sqrt[1 + u^3]], t] /.
Solve[x == u, t]]
(*(2 (2 + (-1)^(2/3)) Sqrt[((1 + (-1)^(2/3) - x) (1 + x))/(2 + (-1)^(
2/3))^2] Sqrt[(-1 + (-1)^(1/3) + x)/(-2 + (-1)^(1/3))]
EllipticF[ArcSin[Sqrt[(1 + x)/(2 + (-1)^(2/3))]],
1/2 (1 + I Sqrt[3])])/Sqrt[1 + x^3]*)
I’m sure one can find a transformation that gives the simplest result, but I don’t know how (