[Octave-bug-tracker] [bug #59776] pcg() returns incorrect flag

[anonymous]

URL:
  <https://savannah.gnu.org/bugs/?59776>

                 Summary: pcg() returns incorrect flag
                 Project: GNU Octave
            Submitted by: None
            Submitted on: Tue 29 Dec 2020 01:24:58 PM UTC
                Category: Octave Function
                Severity: 3 - Normal
                Priority: 5 - Normal
              Item Group: Incorrect Result
                  Status: None
             Assigned to: None
         Originator Name: S Mahesh
        Originator Email: smahesh@iitm.ac.in
             Open/Closed: Open
                 Release: 5.2.0
         Discussion Lock: Any
        Operating System: GNU/Linux

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Details:

In octave 5.2.0, pcg() returns the wrong flag, even though it correctly
computes the answer. MWE: 


A = [1.00000000  -0.00054274  -0.00066848; -0.00054274   1.00000000 
-0.00060330; -0.00066848  -0.00060330   1.00000000];
b = [1 1 1]';
[x, flag, relres, iter] = pcg( A, b, 1e-6, 4, [], [], [1; 1; 1] )


output: 
x =

   1.0012
   1.0011
   1.0013

flag =  1
relres =  0.000025738
iter =  1

The answer (x) is correct: norm(A*x-b) = 0.000000053976 is less than the
tolerance of 1e-6

flag = 1 is supposed to indicate the number of iterations reached the maximum
allowable, but that has not happened here. 

I therefore suspect the value returned in FLAG is a bug. In octave 4.0.0 flag
= 0 is returned correctly. 

Thanks for any help. 





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