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MathGroup Archive 1999

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Re: bug in Legendre polinomials

  • To: mathgroup at smc.vnet.net
  • Subject: [mg20359] Re: [mg20346] bug in Legendre polinomials
  • From: David Withoff <withoff at wolfram.com>
  • Date: Sun, 17 Oct 1999 02:45:36 -0400
  • Sender: owner-wri-mathgroup at wolfram.com

> Hi,
>
> I have found a misterious bug (Version 4.0.1.0):
>
> I have checked the identity:
> Sum[Binomial[ktmp, i]^2 x^i, {i, 0, ktmp}] = (1 - x)^ktmp LegendreP[ktmp,
> (x + 1)/(1 - x)]
> 
> using:
> 
> Simplify[Sum[
>       Binomial[ktmp, i]^2 x^i, {i, 0, ktmp}] - (1 - x)^ktmp LegendreP[
>         ktmp, (x + 1)/(1 - x)]]
> 
> which should be zero for arbitrary ktmp integers.
> Mathematica gives only for ktmp<36 the correct result
> for ktmp>=36 it gives a nonvanishing polinom.
> 
> Thanks
> Peter

I tried a few examples and I always got zero for the expression that
you mentioned, including for ktmp>=36:

In[1]:= expr = Simplify[Sum[
              Binomial[ktmp, i]^2 x^i, {i, 0, ktmp}] - (1 - x)^ktmp LegendreP[
                ktmp, (x + 1)/(1 - x)]]

Out[1]= Hypergeometric2F1[-ktmp, -ktmp, 1, x] - 
 
            ktmp                 1 + x
>    (1 - x)     LegendreP[ktmp, -----]
                                 1 - x

In[2]:= Together[expr /. ktmp -> 60]

Out[2]= 0

If you haven't already gotten an explanation, could you perhaps describe
more explicitly what you did?

Dave Withoff
Wolfram Research


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