[Octave-bug-tracker] [bug #55885] Wrong sparse LU permutation matrix with vector option

[CH]

URL:
  <https://savannah.gnu.org/bugs/?55885>

                 Summary: Wrong sparse LU permutation matrix with vector
option
                 Project: GNU Octave
            Submitted by: atcl
            Submitted on: Mon 11 Mar 2019 02:07:00 PM UTC
                Category: Octave Function
                Severity: 3 - Normal
                Priority: 5 - Normal
              Item Group: Incorrect Result
                  Status: None
             Assigned to: None
         Originator Name: 
        Originator Email: 
             Open/Closed: Open
         Discussion Lock: Any
                 Release: 5.1.0
        Operating System: Any

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Details:

Using the LU decomposition with four output values and the vector option seems
to lead to incorrect results for the P (row permutation) matrix:



A = sprand(5,5,0.3);

[L,U,P,Q] = lu(A);

(mod(find(P),size(P,2)))' % zeros are 5s
(mod(find(Q),size(Q,1)))' % zeros are 5s

[l,u,p,q] = lu(A,'vector');

p'
q'



gives with Octave 5.1



ans =

   1   4   2   3   0

ans =

   0   1   4   3   2

ans =

   1   3   4   2   5

ans =

   5   1   4   3   2



It seems that the vector is created from P' (transposed) instead of P since



(mod(find(P'),size(P,2)))' % zeros are 5s



gives



ans =

   1   3   4   2   0



I hope I did not misinterpret anything. Thank you for your help.

BTW: Matlab (2018a) returns row vectors for P and Q while Octave (5.1) returns
column vectors.





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