Re: [Help-gsl] Underflow in gsl_sf_bessel_Jn_array

[Brian Gough]

At Fri, 18 Jul 2008 12:14:10 +0100,
Jonny Taylor wrote:
> The following is sufficient to demonstrate the issue.
>       double besselArray[51];
>       double rho = 1e-6;
>       gsl_sf_bessel_Jn_array(0, 50, rho, besselArray);
> 
> The problem is that the array is populated by downward recurrence, but  
> J_51 and J_50 are way smaller than DBL_MIN, resulting in an error  
> within the GSL library:
> gsl: gamma.c:1454: ERROR: underflow
> Default GSL error handler invoked.

I looked at the code and there's no easy way to add support for
scaling the results in GSL itself at the moment.  I would recommend
that you make an estimate of n_max using the asymptotic formula for Jn.

-- 
Brian Gough

GNU Scientific Library -
http://www.gnu.org/software/gsl/