dpot02.f

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      SUBROUTINE dpot02( UPLO, N, NRHS, A, LDA, X, LDX, B, LDB, RWORK,
     $                   resid )
*
*  -- LAPACK test routine (version 3.1) --
*     Univ. of Tennessee, Univ. of California Berkeley and NAG Ltd..
*     November 2006
*
*     .. Scalar Arguments ..
      CHARACTER          uplo
      INTEGER            lda, ldb, ldx, n, nrhs
      DOUBLE PRECISION   resid
*     ..
*     .. Array Arguments ..
      DOUBLE PRECISION   a( lda, * ), b( ldb, * ), rwork( * ),
     $                   x( ldx, * )
*     ..
*
*  Purpose
*  =======
*
*  DPOT02 computes the residual for the solution of a symmetric system
*  of linear equations  A*x = b:
*
*     RESID = norm(B - A*X) / ( norm(A) * norm(X) * EPS ),
*
*  where EPS is the machine epsilon.
*
*  Arguments
*  =========
*
*  UPLO    (input) CHARACTER*1
*          Specifies whether the upper or lower triangular part of the
*          symmetric matrix A is stored:
*          = 'U':  Upper triangular
*          = 'L':  Lower triangular
*
*  N       (input) INTEGER
*          The number of rows and columns of the matrix A.  N >= 0.
*
*  NRHS    (input) INTEGER
*          The number of columns of B, the matrix of right hand sides.
*          NRHS >= 0.
*
*  A       (input) DOUBLE PRECISION array, dimension (LDA,N)
*          The original symmetric matrix A.
*
*  LDA     (input) INTEGER
*          The leading dimension of the array A.  LDA >= max(1,N)
*
*  X       (input) DOUBLE PRECISION array, dimension (LDX,NRHS)
*          The computed solution vectors for the system of linear
*          equations.
*
*  LDX     (input) INTEGER
*          The leading dimension of the array X.   LDX >= max(1,N).
*
*  B       (input/output) DOUBLE PRECISION array, dimension (LDB,NRHS)
*          On entry, the right hand side vectors for the system of
*          linear equations.
*          On exit, B is overwritten with the difference B - A*X.
*
*  LDB     (input) INTEGER
*          The leading dimension of the array B.  LDB >= max(1,N).
*
*  RWORK   (workspace) DOUBLE PRECISION array, dimension (N)
*
*  RESID   (output) DOUBLE PRECISION
*          The maximum over the number of right hand sides of
*          norm(B - A*X) / ( norm(A) * norm(X) * EPS ).
*
*  =====================================================================
*
*     .. Parameters ..
      DOUBLE PRECISION   zero, one
      parameter( zero = 0.0d+0, one = 1.0d+0 )
*     ..
*     .. Local Scalars ..
      INTEGER            j
      DOUBLE PRECISION   anorm, bnorm, eps, xnorm
*     ..
*     .. External Functions ..
      DOUBLE PRECISION   dasum, dlamch, dlansy
      EXTERNAL           dasum, dlamch, dlansy
*     ..
*     .. External Subroutines ..
      EXTERNAL           dsymm
*     ..
*     .. Intrinsic Functions ..
      INTRINSIC          max
*     ..
*     .. Executable Statements ..
*
*     Quick exit if N = 0 or NRHS = 0.
*
      IF( n.LE.0 .OR. nrhs.LE.0 ) THEN
         resid = zero
         return
      END IF
*
*     Exit with RESID = 1/EPS if ANORM = 0.
*
      eps = dlamch( 'Epsilon' )
      anorm = dlansy( '1', uplo, n, a, lda, rwork )
      IF( anorm.LE.zero ) THEN
         resid = one / eps
         return
      END IF
*
*     Compute  B - A*X
*
      CALL dsymm( 'Left', uplo, n, nrhs, -one, a, lda, x, ldx, one, b,
     $            ldb )
*
*     Compute the maximum over the number of right hand sides of
*        norm( B - A*X ) / ( norm(A) * norm(X) * EPS ) .
*
      resid = zero
      DO 10 j = 1, nrhs
         bnorm = dasum( n, b( 1, j ), 1 )
         xnorm = dasum( n, x( 1, j ), 1 )
         IF( xnorm.LE.zero ) THEN
            resid = one / eps
         ELSE
            resid = max( resid, ( ( bnorm / anorm ) / xnorm ) / eps )
         END IF
   10 continue
*
      return
*
*     End of DPOT02
*
      END