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MathGroup Archive 2005

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Re: Problem with evaluation of Besel Functions

  • To: mathgroup at smc.vnet.net
  • Subject: [mg56064] Re: Problem with evaluation of Besel Functions
  • From: "adamizer" <adam.smith at hillsdale.edu>
  • Date: Thu, 14 Apr 2005 08:56:18 -0400 (EDT)
  • References: <d3g8kd$smh$1@smc.vnet.net>
  • Sender: owner-wri-mathgroup at wolfram.com

Many have noted the difference between using 3/2 vs 1.5

However there is another aspect to your post that I noticed.  The
result depends on exactly how you do the substitution.  Reversing the
order avoids the complex infinity, indeterminate problem.

Look at:
(BesselJ[xx, yyy] /. xx -> 1.5) /. yyy -> 0
Returns ComplexInfinity (or Inderterminate for xx->3/2)

versus doing it in the other order:
(BesselJ[xx, yyy] /. yyy -> 0) /. xx -> 1.5
Returns 0.  (or 0 for xx->3/2)

A technique that I often employ is to substitute all variables at the
same time by enclosing all the replacements inside curly brackets.

BesselJ[xx, yyy]/.{xx->1.5,yyy->0.}

This may help with your "problems with Bessel and Legendre" function
evaluating differently.

Adam Smith



Ariel sumeruk wrote:
> Hello
> I am having a problem evaluating various functions, One example is
the following
>
> BesselJ[1.5, 0] evaluates to 0 but
> (BesselJ[xx, yyy] /. xx -> 1.5) /. yyy -> 0 evaluates to complex
infinity
>
> I seem to encounter many of these problems with Bessel and Legendre
> functions where I get actual diffrent numerical results depending on
> How I set the parameters.
> Thanks for anyone who might help
> Ariel


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