[Octave-bug-tracker] [bug #60839] linsolve returns an incorrect solution with an inconsistent system

[Markus Mützel]

Update of bug #60839 (project octave):

                  Status:                    None => Invalid                
             Open/Closed:                    Open => Closed                 

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Follow-up Comment #1:

See the documentation for `linsolve`:
>> With no options, this function is equivalent to the left division operator
(x = A \ b) or the matrix-left-divide function (x = mldivide (A, b)). 
>> The optional second output R is the inverse condition number of A (zero if
matrix is singular). 

And the documentation for `mldivide`:
>> If the system is not square, or if the coefficient matrix is singular, a
minimum norm solution is computed. 

So what you are seeing is the expected result afaict.

You could check the second output argument of `linsolve` to query whether the
input matrix is singular.
For your example:

>> [S, R] =
linsolve([1.000,-1.000,1.000,1.000,-1.000;1.000,-1.000,-1.000,1.000,-1.000;1.000,-1.000,-1.000,1.000,-1.000;-1.000,-1.000,1.000,1.000,-1.000;-1.000,-1.000,1.000,-1.0
00,-1.000],[-1.000;-1.000;1.000;1.000;1.000])
warning: matrix singular to machine precision
warning: called from
    linsolve at line 113 column 7

S =

  -1.0000e+00
  -2.5000e-01
  -5.0000e-01
  -2.5622e-16
  -2.5000e-01

R = 0



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