# Finding function from given sequence?

I only have six numbers, $\lbrace4, 48, 360, 2240, 12600, 66528\rbrace$, from the following series:

Table[1/SeriesCoefficient[ArcSin[x/2]^2, {x, 0, 2 n}], {n, 1, 6}]


I want to find a function $f(x)$ such that

$$\sum_{n=1}^{\infty}f(n)x^{2n}=\arcsin^2 \bigg(\frac{x}{2}\bigg)$$

The is a Maple code to find it

with(gfun)

s := [seq(1/coeff(series(arcsin((1/2)*x)^2, x, 25), x, 2*n), n = 1 .. 12)]

R := unapply(rsolve(op(1, listtorec(s, r(m))), r(m)), m); [seq(R(m), m = 0 .. 8)] How do I find it using Mathematica?

• Assuming[n >= 1, 1/SeriesCoefficient[ArcSin[Sqrt[x]/2]^2, {x, 0, n}]] yields a rather complicated expression. – J. M.'s technical difficulties Oct 4 '16 at 13:03
• The left side of your equation is independent of x, but the right side is not. Do you mean Sum[f[n] x^(2 n), {n, 1, Infinity}]? – bbgodfrey Oct 4 '16 at 13:08

The problem with the result of SeriesCoefficient[] is that the hypergeometric functions that show up in it are left unsimplified:

Assuming[n >= 0, FullSimplify[1/SeriesCoefficient[ArcSin[Sqrt[x]/2]^2/x, {x, 0, n}]]]
(2^(3 + 2 n) (1 + 2 n) Sqrt[π] Gamma[2 + n])/
(Gamma[1/2 + n] ((1 + 2 n) HypergeometricPFQ[{1/2, 1/2, -n}, {3/2, 1/2 - n}, 1] +
HypergeometricPFQ[{1/2, -(1/2) - n, -n}, {1/2 - n, 1/2 - n}, 1]))


Note, however, that since n is assumed to be a positive integer, the hypergeometric functions in it are actually terminating hypergeometric series; that is, one of the Pochhammer factors in the term becomes zero past a certain index. In particular, we can make the replacements

HypergeometricPFQ[{1/2, 1/2, -n}, {3/2, 1/2 - n}, 1] ->
Sum[(Pochhammer[1/2, k]^2 (-1)^k Binomial[n, k])/
(Pochhammer[3/2, k] Pochhammer[1/2 - n, k]), {k, 0, n}]

HypergeometricPFQ[{1/2, -1/2 - n, -n}, {1/2 - n, 1/2 - n}, 1] ->
Sum[(Pochhammer[1/2, k] Pochhammer[-1/2 - n, k] (-1)^k Binomial[n, k])/
Pochhammer[1/2 - n, k]^2, {k, 0, n}]


which respectively evaluate to (π (n!)^2)/(2 (1/2 (-1 + 2 n))! (1/2 (1 + 2 n))!) and (π (n!)^2)/((1/2 (-1 + 2 n))!)^2; if we plug these into the original expression returned by SeriesCoefficient[], we have

FullSimplify[(2^(2 n + 3) (2 n + 1) Sqrt[π] Gamma[2 + n])/
(Gamma[1/2 + n] ((2 n + 1) π (n!)^2/(2 ((2 n - 1)/2)! ((2 n + 1)/2)!) +
π (n!)^2/(((2 n - 1)/2)!)^2))]
(2^(3 + 2 n) (1 + n) Gamma[3/2 + n])/(Sqrt[π] Gamma[1 + n])


The result can of course be further simplified to 2^(2 n + 2) (n + 1) Pochhammer[3/2, n]/n!.