bugGNU Octave - Bugs: bug #62048, acos and cos are giving different...

 
 

bug #62048: acos and cos are giving different answers

Submitter:  None
Submitted:  Sun 13 Feb 2022 03:24:53 PM UTC
   
 
Category:  Octave Function Severity:  3 - Normal
Priority:  5 - Normal Item Group:  Incorrect Result
Status:  Invalid / Not an Octave Bug Assigned to:  None
Originator Name:  Originator Email:  -email is unavailable-
Open/Closed:  * Closed Release:  * 6.4.0
Operating System:  * Any Fixed Release:  None
Planned Release:  None
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Sun 13 Feb 2022 04:23:46 PM UTC, comment #1: 

I think, you can not expect that both loops converge to the equilibrium xe = 0.7391. Since |cos(xe)'| < 1 and |acos(xe)'| > 1, the loop with cos() converges and the loop with acos() diverges. You can observe the same for x = x^2 and x = sqrt(x) when starting at x near to the equilibrium xe = 1.

Closing this report as invalid.


Torsten Lilge <ttl>
Group Member
Sun 13 Feb 2022 03:24:53 PM UTC, original submission:  


>> x = 0.74; for i = 1:100000, x = cos(x); end; x
x = 0.7391
>> x = 0.74; for i = 1:100000, x = acos(x); end; x
x = 0.9623 - 1.1110i


Anonymous

 

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    Date Changed by Updated Field Previous Value => Replaced by
    2022-02-13 ttl StatusNone Invalid / Not an Octave Bug
        Open/ClosedOpen Closed

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