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[Lapack] Error in the reference implementation of DGEMV

Hello BLAS/LAPACK people

 

I think I have found an error in the reference implementation of DGEMV.
The error appears in case N = 0 and BETA != 1.0. The following simple
program demonstrates the error

 

      PROGRAM TEST

      DOUBLE PRECISION A(3,0),X(0),Y(3),ALPHA,BETA

      DATA Y/1.0,2.0,3.0/

      DATA ALPHA/1.0/

      DATA BETA/2.0/

      WRITE (6,*) 'Before: Y = [',Y(1),',',Y(2),',',Y(3),']'

      CALL DGEMV('N',3,0,ALPHA,A,3,X,1,BETA,Y,1)

      WRITE (6,*) 'After:  Y = [',Y(1),',',Y(2),',',Y(3),']'

      END

 

Before calling DGEMV, we obviously get Y = [1.0, 2.0, 3.0], whereas I
would expect Y = [2.0, 4.0, 6.0] after. However the test shows that Y is
unchanged.

 

Looking into the reference implementation of DGEMV it seems that N = 0
always causes a quick return, which I think is an error in case BETA !=
1.0. The problem is also present in the routine DGBMV, and of cause in
the corresponding single precision versions.

 

Thank you for doing such a wonderful job on linear algebra. This highly
appreciated and very useful.

 

 

Best regards

Erik Nim 

 

 

 

Erik Nim 
Garrad Hassan Denmark ApS
Pakhusvej 3, 1st Floor, DK-8382 Hinnerup, Denmark
Tel: +45 87 65 61 11 / Fax: +45 87 65 61 13 / Mobile: +45 21 65 61 08 
www.garradhassan.com 

 

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