[Octave-bug-tracker] [bug #47236] output of eigs() with 'sa' option differs from Matlab

[Mike Miller]

Follow-up Comment #6, bug #47236 (project octave):

Can you show what the results are and what they should be? I'm assuming it's
the sort order of the d diagonal matrix, but the values are correct?


octave:1> load yaledata 
octave:2> L = diag (sum (A)) - A;
octave:3> [v, m] = eigs (A, 3, "sa"); diag (m), eig (A)(1:3)
ans =

  -3.5256
  -3.5358
  -3.9126

ans =

  -3.9126
  -3.5358
  -3.5256

octave:4> [v, d] = eigs (L, 3, "sa"); diag (d), eig (L)(1:3)
ans =

   2.5453e-02
   2.0710e-02
   2.9696e-15

ans =

   3.0197e-14
   2.0710e-02
   2.5453e-02




Note that even Matlab's help for eigs claims that "eigs does not always return
sorted eigenvalues and eigenvectors. Use sort to explicitly sort the output
eigenvalues and eigenvectors in cases where their order is important." (from
https://www.mathworks.com/help/matlab/ref/eigs.html)

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