MathGroup Archive 2005

[Date Index] [Thread Index] [Author Index]

Search the Archive

Re: Problem with evaluation of Besel Functions

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

Using approximate arithmetic near singularities can have such 
consequences. This follows from the fact that the limiting value for a 
multivariate function is path dependent; small perturbations might make 
a denominator vanish while a numerator is "small" but not manifestly 
zero. That's pretty much what happens in this example.

If you use xx->3/2 you will (appropriately) obtain Indeterminate due to 
a 0/0 situation. Even this is not always going to work as you might 
like, because in general numerator or denominator might be a "disguised" 
zero while the other one is explicit, causing a result of zero or infinity.

Daniel Lichtblau
Wolfram Research

  • Prev by Date: Re: Numerical Optimization involving equation solving
  • Next by Date: Re: Re: Re: Re: ! operator
  • Previous by thread: Problem with evaluation of Besel Functions
  • Next by thread: Re: Problem with evaluation of Besel Functions