LAPACK Archives

[Lapack] matrix splitting problem with dstebz and sstebz


Greetings,

We have discovered a problem with DSYEVX.  The problem occurs for certain
matrices created using a specific  algorithm.

We are able to work around the problem by removing the "matrix splitting"
code in dstebz.  This problem also occurs in sstebz.

      DO 10 J = 2, N
         TMP1 = E( J-1 )**2
!!       IF( ABS( D( J )*D( J-1 ) )*ULP**2+SAFEMN.GT.TMP1 ) THEN
!!          ISPLIT( NSPLIT ) = J - 1
!!          NSPLIT = NSPLIT + 1
!!          WORK( J-1 ) = ZERO
!!       ELSE
            WORK( J-1 ) = TMP1
            PIVMIN = MAX( PIVMIN, TMP1 )
!!       END IF
   10 CONTINUE

Please see the attached test program testevx.f which demonstrates the
failure.
(See attached file: testevx.f)

Output from the test:.


      *** Math check results for  DSYT22 with N=   7 and M=   5
      *** exceeded the threshold value of            .10000000D+03

      *** Math check:   | U' A U - S | / ( |A| m ulp )
      ***   result was            .76390963D+13
      *** Math check:   | I - U'U | / ( m ulp )
      ***   result was            .15278193D+15


-------------- next part --------------
An HTML attachment was scrubbed...
URL: 
http://lists.cs.utk.edu/private/lapack/attachments/20090629/0b84bc2a/attachment.html
 
-------------- next part --------------
A non-text attachment was scrubbed...
Name: testevx.f
Type: application/octet-stream
Size: 9269 bytes
Desc: not available
Url : 
http://lists.cs.utk.edu/private/lapack/attachments/20090629/0b84bc2a/testevx.obj
 

<Prev in Thread] Current Thread [Next in Thread>


For additional information you may use the LAPACK/ScaLAPACK Forum.
Or one of the mailing lists, or