       Re: Hypergeometric1F1 polynomial

• To: mathgroup at smc.vnet.net
• Subject: [mg91470] Re: Hypergeometric1F1 polynomial
• From: "Alec Mihailovs" <alec at mihailovs.com>
• Date: Sat, 23 Aug 2008 01:40:24 -0400 (EDT)

```From: "Jean-Marc Gulliet" <jeanmarc.gulliet at gmail.com>
> On Fri, Aug 22, 2008 at 7:13 AM, Alec Mihailovs <alec at mihailovs.com>
> wrote:
>> The problem is that the answers given by Mathematica to the Sum problem,
>> are
>> not the same - they are not polynomials, with the series expansion, or
>> without.
>
> Hum, with series expansion they are (at least on my system). For instance,
>
> In:= s = Sum[Binomial[n, k]/Binomial[2 n, k]/k! (2 x)^k, {k, 0, n}]
>
> Out=
>
> -(1/2) - n  x  1/2 + n         1                      1
> 2           E  x        BesselI[- (-1 - 2 n), x] Gamma[- - n]
>                                2                      2
>
> In:= Table[Series[s, {x, 0, n}] // Normal, {n, 0, 5}] // TableForm

That just tells that the beginning of the series is the same. But the rest
should be subtracted to get the correct answer - that's generally how the
answer with BesselK appears - as a result of subtracting of 2 expressions
with BesselI.

> In:= FullSimplify[s]
> Table[Series[%, {x, 0, n}] // Normal, {n, 0, 5}] // TableForm

The same here - if you took more than n terms from the series, you would see
the difference. 1 is not equal to E^(2x) even if the first term of Taylor
series is the same :)

> In:= Table[FullSimplify[s] == Hypergeometric1F1[-n, -2 n, 2 x], {n,
> 0, 5}, {x, 1,
>  5}]
>
> Out= {{False, False, False, False, False}, {False, False, False, False,
>  False}, {False, False, False, False, False}, {False, False, False, False,
>  False}, {False, False, False, False, False}, {False, False, False, False,
>  False}}

That, certainly, is correct - the answer given by Mathematica is not equal
to the correct answer (as I said in the original post).

Alec

```

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