sspt01.f

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      SUBROUTINE SSPT01( UPLO, N, A, AFAC, IPIV, C, LDC, 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            LDC, N
      REAL               RESID
*     ..
*     .. Array Arguments ..
      INTEGER            IPIV( * )
      REAL               A( * ), AFAC( * ), C( LDC, * ), RWORK( * )
*     ..
*
*  Purpose
*  =======
*
*  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.
*
*  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.
*
*  A       (input) REAL array, dimension (N*(N+1)/2)
*          The original symmetric matrix A, stored as a packed
*          triangular matrix.
*
*  AFAC    (input) 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.
*
*  IPIV    (input) INTEGER array, dimension (N)
*          The pivot indices from SSPTRF.
*
*  C       (workspace) REAL array, dimension (LDC,N)
*
*  LDC     (integer) INTEGER
*          The leading dimension of the array C.  LDC >= max(1,N).
*
*  RWORK   (workspace) REAL array, dimension (N)
*
*  RESID   (output) 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 )
*
*  =====================================================================
*
*     .. 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