DLAHQR convergence problem

[yokimi]

Hi LAPACK developers,
I encountered a convergence problem of DLAHQR in V.3.5.0. However V.2.0 converged for the same problem.
I found a difference between two versions as shown below.

in Version2.0
NH = IHI - ILO + 1
ITN = 30*NH
DO 130 ITS = 0, ITN

in Version3.5.0
PARAMETER ( ITMAX = 30 )
DO 140 ITS = 0, ITMAX

ITN and ITMAX are the maximum number of QR iterations.
Modified V.3.5.0,where ITMAX is changed to ITMAX*NH,converged for the same problem.
Is this a bug or intentionally changed ?

regards,
Yoshihide Okimi

[admin]

Hi, sorry for the late answer.
What is the V2.0 that you are comparing too?
LAPACK 3.1.0 already had that change.
Julie

[yokimi]

Hi, Julie. Thank you for your reply.

The V2.0 is enclosed in ARPACK (ARPACK96) developed by Rice University.
I encountered convergence failures in eigenvalue problems using ARPACK with the latest version of LAPACK. In case that DLAHQR enclosed in ARPACK is linked, there are no problems.
Comparing the two versions, I found the difference of the maximum number of QR iterations.

yokimi

[Julien Langou]

Hi Yoshihide, thanks for letting us know about this problem. This was flagged as bug 125 in the LAPACK errata, and a patch (essentially what you are suggesting) was applied SVN rev1537. Thanks a lot! Julien.

Related URL:
http://www.netlib.org/lapack/Errata/index2.html#_strong_span_class_green_bug125_span_strong_xlahqr_v3_5_does_not_converge_on_some_matrices_while_xlahqr_v2_converges