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Re: [Help-gsl] integration w/ qagp, bessel_Jnu

From: Jorge Talamantes
Subject: Re: [Help-gsl] integration w/ qagp, bessel_Jnu
Date: Thu, 17 Nov 2005 12:21:58 -0800

>Split up the integral (or integrand) to compute the part near x=0
>using the asymptotic form of j(x,n) for small x to avoid underflow.
>Or disable the underflow error if it doesn't affect the final results.
>Brian Gough

Hi. Thanks for your reply.  I implemented a solution equivalent to your
second suggestion: it turns out that I can prevent gnu_sf_bessel_Jnu(x,n)
from collapsing with an underflow error if I ensure that (n-x)<~70. (n, x
> 0.) I did this by setting the integrand equal to zero if this condition
is not satisfied. Then I checked my integral for a few relevant values of
n and x. (I used matlab for cross-checking.) The integral did not change.
Thanks again.


Jorge Talamantes, Physics and Geology, California State University,
Bakersfield, CA 93311, jtalamantes at csub dot edu, 661.654.2335

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