sspt01.f

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*> \brief \b SSPT01
*
*  =========== DOCUMENTATION ===========
*
* Online html documentation available at
*            http://www.netlib.org/lapack/explore-html/
*
*  Definition:
*  ===========
*
*       SUBROUTINE SSPT01( UPLO, N, A, AFAC, IPIV, C, LDC, RWORK, RESID )
*
*       .. Scalar Arguments ..
*       CHARACTER          UPLO
*       INTEGER            LDC, N
*       REAL               RESID
*       ..
*       .. Array Arguments ..
*       INTEGER            IPIV( * )
*       REAL               A( * ), AFAC( * ), C( LDC, * ), RWORK( * )
*       ..
*
*
*> \par Purpose:
*  =============
*>
*> \verbatim
*>
*> SSPT01 reconstructs a symmetric indefinite packed matrix A from its
*> block L*D*L' or U*D*U' factorization and computes the residual
*>      norm( C - A ) / ( N * norm(A) * EPS ),
*> where C is the reconstructed matrix and EPS is the machine epsilon.
*> \endverbatim
*
*  Arguments:
*  ==========
*
*> \param[in] UPLO
*> \verbatim
*>          UPLO is CHARACTER*1
*>          Specifies whether the upper or lower triangular part of the
*>          symmetric matrix A is stored:
*>          = 'U':  Upper triangular
*>          = 'L':  Lower triangular
*> \endverbatim
*>
*> \param[in] N
*> \verbatim
*>          N is INTEGER
*>          The number of rows and columns of the matrix A.  N >= 0.
*> \endverbatim
*>
*> \param[in] A
*> \verbatim
*>          A is REAL array, dimension (N*(N+1)/2)
*>          The original symmetric matrix A, stored as a packed
*>          triangular matrix.
*> \endverbatim
*>
*> \param[in] AFAC
*> \verbatim
*>          AFAC is REAL array, dimension (N*(N+1)/2)
*>          The factored form of the matrix A, stored as a packed
*>          triangular matrix.  AFAC contains the block diagonal matrix D
*>          and the multipliers used to obtain the factor L or U from the
*>          block L*D*L' or U*D*U' factorization as computed by SSPTRF.
*> \endverbatim
*>
*> \param[in] IPIV
*> \verbatim
*>          IPIV is INTEGER array, dimension (N)
*>          The pivot indices from SSPTRF.
*> \endverbatim
*>
*> \param[out] C
*> \verbatim
*>          C is REAL array, dimension (LDC,N)
*> \endverbatim
*>
*> \param[in] LDC
*> \verbatim
*>          LDC is INTEGER
*>          The leading dimension of the array C.  LDC >= max(1,N).
*> \endverbatim
*>
*> \param[out] RWORK
*> \verbatim
*>          RWORK is REAL array, dimension (N)
*> \endverbatim
*>
*> \param[out] RESID
*> \verbatim
*>          RESID is REAL
*>          If UPLO = 'L', norm(L*D*L' - A) / ( N * norm(A) * EPS )
*>          If UPLO = 'U', norm(U*D*U' - A) / ( N * norm(A) * EPS )
*> \endverbatim
*
*  Authors:
*  ========
*
*> \author Univ. of Tennessee
*> \author Univ. of California Berkeley
*> \author Univ. of Colorado Denver
*> \author NAG Ltd.
*
*> \ingroup single_lin
*
*  =====================================================================
      SUBROUTINE sspt01( UPLO, N, A, AFAC, IPIV, C, LDC, RWORK, RESID )
*
*  -- LAPACK test routine --
*  -- LAPACK is a software package provided by Univ. of Tennessee,    --
*  -- Univ. of California Berkeley, Univ. of Colorado Denver and NAG Ltd..--
*
*     .. Scalar Arguments ..
      CHARACTER          UPLO
      INTEGER            LDC, N
      REAL               RESID
*     ..
*     .. Array Arguments ..
      INTEGER            IPIV( * )
      REAL               A( * ), AFAC( * ), C( LDC, * ), RWORK( * )
*     ..
*
*  =====================================================================
*
*     .. Parameters ..
      REAL               ZERO, ONE
      parameter( zero = 0.0e+0, one = 1.0e+0 )
*     ..
*     .. Local Scalars ..
      INTEGER            I, INFO, J, JC
      REAL               ANORM, EPS
*     ..
*     .. External Functions ..
      LOGICAL            LSAME
      REAL               SLAMCH, SLANSP, SLANSY
      EXTERNAL           lsame, slamch, slansp, slansy
*     ..
*     .. External Subroutines ..
      EXTERNAL           slavsp, slaset
*     ..
*     .. Intrinsic Functions ..
      INTRINSIC          real
*     ..
*     .. Executable Statements ..
*
*     Quick exit if N = 0.
*
      IF( n.LE.0 ) THEN
         resid = zero
         RETURN
      END IF
*
*     Determine EPS and the norm of A.
*
      eps = slamch( 'Epsilon' )
      anorm = slansp( '1', uplo, n, a, rwork )
*
*     Initialize C to the identity matrix.
*
      CALL slaset( 'Full', n, n, zero, one, c, ldc )
*
*     Call SLAVSP to form the product D * U' (or D * L' ).
*
      CALL slavsp( uplo, 'Transpose', 'Non-unit', n, n, afac, ipiv, c,
     $             ldc, info )
*
*     Call SLAVSP again to multiply by U ( or L ).
*
      CALL slavsp( uplo, 'No transpose', 'Unit', n, n, afac, ipiv, c,
     $             ldc, info )
*
*     Compute the difference  C - A .
*
      IF( lsame( uplo, 'U' ) ) THEN
         jc = 0
         DO 20 j = 1, n
            DO 10 i = 1, j
               c( i, j ) = c( i, j ) - a( jc+i )
   10       CONTINUE
            jc = jc + j
   20    CONTINUE
      ELSE
         jc = 1
         DO 40 j = 1, n
            DO 30 i = j, n
               c( i, j ) = c( i, j ) - a( jc+i-j )
   30       CONTINUE
            jc = jc + n - j + 1
   40    CONTINUE
      END IF
*
*     Compute norm( C - A ) / ( N * norm(A) * EPS )
*
      resid = slansy( '1', uplo, n, c, ldc, rwork )
*
      IF( anorm.LE.zero ) THEN
         IF( resid.NE.zero )
     $      resid = one / eps
      ELSE
         resid = ( ( resid / real( n ) ) / anorm ) / eps
      END IF
*
      RETURN
*
*     End of SSPT01
*
      END