Why compute value for nmax+1 in bessel_Jn_array()?
Ruslan Kabatsayev
b7.10110111@gmail.com
Fri Jun 5 07:30:00 GMT 2015
Hello,
I'm looking into implementation of gsl_sf_bessel_Jn_array(). There
stat_np1 is computed for nmax+1 and stat_n for nmax, so that later the
backward recurrence relation is used to obtain values for nmin through
nmax-1. It seems strange to me though that nmax+1 is used at all —
can't the function just go down from nmax and nmax-1 instead of nmax+1
and nmax? It seems currently it does one extra computation only to
effectively throw away its result. This also becomes a problem e.g.
when x==0.7 and nmax==141: the underflow error makes it fail, although
if the function started from nmax instead of nmax+1, it'd succeed as
it currently does for nmax==140.
Could someone clarify this to me?
Regards,
Ruslan
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