From bug-octave-request at bevo dot che dot wisc dot edu  Mon Jun  4 10:57:08 2001
Subject: besseli output range apparently limited to about 10^33
From: Daniel Alt <daniel dot alt at ttpcom dot com>
To: bug-octave at bevo dot che dot wisc dot edu
Date: Mon, 4 Jun 2001 10:56:59 -0500


"Bug" report for Octave 2.1.31 configured for MS Windows
(octave-windows-2000oct25d.exe downloaded 1 June 2001 from
sourceforge.net)

(I attempted, unsuccessfully, to use Octave's bug_report command to
generate this email.  My PC doesn't have Unix shell command emulators
installed, so bug_report fails with "not found" errors for grep, cat,
chmod, etc.)

It might be stretching the definition to call this a bug, but besseli's
output goes to infinity at around 10^33 (instead of at the maximum
floating point value of about 10^308).  Presumably this is because some
intermediate result exceeds the floating point range, but both Matlab
and Microsoft Excel can calculate besseli values of at least 10^300.

Here is a bit of an Octave transcript showing the transition:

octave:87> besseli(19,81.3565)
ans =  1.0327e+33
octave:88> besseli(19,81.357)
ans = Inf + Infi

As one might expect, besselj has similar behaviour for equivalent
inputs:

octave:92> besselj(19,81.357i)
ans = Inf + Infi
octave:93> besselj(19,81.3565i)
ans = -6.3238e+16 - 1.0327e+33i

(One question on this besselj result:  Shouldn't ans be purely imaginary
in the second case?)



Regards,

Dan Alt



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