[Octave-bug-tracker] [bug #57185] svds returns inconsistent singular values

[Rik]

Follow-up Comment #7, bug #57185 (project octave):

I think we should consider fixing this is two steps.  The first question to
resolve is the return of complex singular values rather than just real ones. 
The complex values are very small, which suggests the algorithm is effective
but there is a small loss of precision owing to the fact that this is imperfct
numerical computation.  Sample:


octave:9>  x = randn(10, 5) ;
octave:10> y = randn(10, 5) ;
octave:11> z = complex(x, y) ;
octave:12> s = svds (z)
s =

   5.9713 + 0.0000i
   4.6205 - 0.0000i
   3.7797 + 0.0000i
   2.5995 + 0.0000i
   1.9744 - 0.0000i

octave:13> imag (s)
ans =

   9.0733e-17
  -1.2895e-16
   9.0859e-17
   2.8342e-19
  -7.4995e-17



A simple answer might be to take the equivalent, in C++, of


s = real (s);


before returning the values.

Alternatively, it make be possible to declare s as a real array in C++ so that
it can never hold imaginary values.


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