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Re: LegendreP error (bug?) in Mathematica

• To: mathgroup at smc.vnet.net
• Subject: [mg81214] Re: LegendreP error (bug?) in Mathematica
• From: Bill Rowe <readnewsciv at sbcglobal.net>
• Date: Sun, 16 Sep 2007 04:02:15 -0400 (EDT)

On 9/15/07 at 4:15 AM, rschmied at gmail.com (Roman) wrote:

>But, instead of telling you that you are getting garbage, the
>algorithm returns a number with exaggerated precision:

>In[3] := LegendreP[200, 43, 0.8] // Precision
>Out[3] = MachinePrecision

As I understand the documentation, expressions with only exact
numbers and machine precision numbers always return
MachinePrecision as the precision. If I have this correct, then
Precision only returns useful results when expressions make use
of Mathematica's variable precision arithmetic.

Instead of Precision using Accuracy

In[5]:= LegendreP[200, 43, 0.8] // Accuracy

Out[5]= -102.834

gives a clear indication what is returned by LegendreP[200, 43,
0.8] is problematic.

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• Follow-Ups:
  • Re: Re: LegendreP error (bug?) in Mathematica
    • From: Andrzej Kozlowski <akoz@mimuw.edu.pl>