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[Octave-bug-tracker] [bug #44310] log and functions which use log return

From: Michael Godfrey
Subject: [Octave-bug-tracker] [bug #44310] log and functions which use log return incorrect result
Date: Thu, 26 Feb 2015 14:52:43 +0000
User-agent: Mozilla/5.0 (X11; Fedora; Linux x86_64; rv:35.0) Gecko/20100101 Firefox/35.0

Follow-up Comment #6, bug #44310 (project octave):

Here is the conclusion of where the atanh error comes
from, and a patch is attached that tests for the error
in mappers.cc.

On FC14 Octave 3.8.1
octave:10> xp/xm
ans = -0.980198019801980 + 0.198019801980198i
octave:11> log(xp/xm)/2
ans =  0.000000000000000 + 1.471127674303735i

On FC14 Octave 3.9.1+
octave:4> xp/xm
ans = -0.980198019801980 + 0.198019801980198i
octave:5> log(xp/xm)/2
ans =  0.000000000000000 + 1.471127674303735i

On FC21 Octave 3.9.1+
octave:14> xp/xm
ans = -0.980198019801980 + 0.198019801980198i
octave:15> log(xp/xm)/2
ans =  4.67173054916526e-17 + 1.47112767430373e+00i

So, the error is in the log function in libm-2.20.
The error was introduced after libm-2.18.

My attempt to communicate this error to the glibc folks
appears not to have gotten through.

Applying the attached patch will at least remind everyone
that the error exists.

(file #33194)

Additional Item Attachment:

File name: complex_atanh_error.diff       Size:0 KB


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