• To: mathgroup at smc.vnet.net
• Subject: [mg25414] LegendreP & Gauss quad bug
• From: Paul Cally <cally at kronos.maths.monash.edu.au>
• Date: Fri, 29 Sep 2000 01:07:11 -0400 (EDT)
• Organization: Monash University
• Sender: owner-wri-mathgroup at wolfram.com

```There is a bug in the V4 implementation of the Legendre function
LegendreP, at
least in the linux version.

I am running Mathematica 4.0 under RedHat 6.2 linux. I have found the
following bug:

Mathematica 4.0 for Linux
-- Motif graphics initialized --

In[1]:= LegendreP[5,2,N[1/3]]

Out[1]= -10.3704

In[2]:= LegendreP[5,2,N[1/3,20]]

Out[2]= -0.17283950617283951

In[3]:= %%/%

Out[3]= 60.

===================================

LegendreP[n,m,x] appears to work properly if x is a machine precision
number, but if it is higher precision, it gives a result which is out by

EXACTLY the factor m!/n! This error is NOT present in Version 3.0
running
under Digital Unix.

Please note that this also causes the standard package
to give wrong results for the quadrature weights if n is odd and
precision
is set to greater than 16.

Out[2]= {{-0.774597, 0.555556}, {0, 0.888889}, {0.774597, 0.555556}}

Out[3]= {{-0.77459666924148337703585, 0.98360655737704918033},

>    {0, 0.0327868852459016393443},

>    {0.77459666924148337703585, 0.98360655737704918033}}

If n is even, there is no abscissa at x=0, and the error scales away.
However, if n is odd, the unscaled weight associated with the exact
number
x=0 is correct but all the others are wrong. When normalized, all the
weights are therefore greatly in error.

Paul Cally

--

+--------------------------------------------------------------------------+
|Assoc Prof Paul Cally            |    Ph:  +61 3 9905-4471                |
|Dept of Mathematics & Statistics |    Fax: +61 3 9905-3867                |
|Monash University                |    paul.cally at sci.monash.edu.au        |
|PO Box 28M, Victoria 3800        |                                        |
|AUSTRALIA                        | http://www.maths.monash.edu.au/~cally/ |
+--------------------------------------------------------------------------+

```

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