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[Octave-bug-tracker] [bug #49091] MinGW std::acosh less accurate than Li

From: Markus Mützel
Subject: [Octave-bug-tracker] [bug #49091] MinGW std::acosh less accurate than Linux versions
Date: Fri, 5 Aug 2022 02:27:27 -0400 (EDT)

Follow-up Comment #26, bug #49091 (project octave):

The negative zero seems to be the cause of the issue indeed. Thank you all for
tracking this down. I didn't know that `-10i` in Octave actually meant
`-(0,10)` as opposed to `(0,-10)`.
With a modified version of the test program in comment #15, I see:

$ ./test-acosh.exe
x = 0.0000000000000000e+00
y = 1.0000000000000000e+01
    cacosh(+x+1i*y) = 2.9982229502979698 + 1.5707963267948966i
    cacosh(+x-1i*y) = 2.9982229502979698 + -1.5707963267948966i
    cacosh(-x-1i*y) = 2.9982229502979760 + -1.5707963267948966i
std::acosh(+x+1i*y) = 2.9982229502979698 + 1.5707963267948966i
std::acosh(+x-1i*y) = 2.9982229502979698 + -1.5707963267948966i
std::acosh(-x-1i*y) = 2.998222950297976 + -1.5707963267948966i

That could probably be solved by using symmetries of the function also for the
default implementation in MinGW's CRT (not only for large input values).


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