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170 lines
4.2 KiB
170 lines
4.2 KiB
*> \brief \b SPTT01
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*
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* =========== DOCUMENTATION ===========
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*
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* Online html documentation available at
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* http://www.netlib.org/lapack/explore-html/
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*
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* Definition:
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* ===========
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*
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* SUBROUTINE SPTT01( N, D, E, DF, EF, WORK, RESID )
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*
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* .. Scalar Arguments ..
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* INTEGER N
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* REAL RESID
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* ..
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* .. Array Arguments ..
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* REAL D( * ), DF( * ), E( * ), EF( * ), WORK( * )
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* ..
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*
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*
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*> \par Purpose:
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* =============
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*>
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*> \verbatim
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*>
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*> SPTT01 reconstructs a tridiagonal matrix A from its L*D*L'
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*> factorization and computes the residual
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*> norm(L*D*L' - A) / ( n * norm(A) * EPS ),
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*> where EPS is the machine epsilon.
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*> \endverbatim
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*
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* Arguments:
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* ==========
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*
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*> \param[in] N
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*> \verbatim
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*> N is INTEGER
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*> The order of the matrix A.
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*> \endverbatim
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*>
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*> \param[in] D
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*> \verbatim
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*> D is REAL array, dimension (N)
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*> The n diagonal elements of the tridiagonal matrix A.
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*> \endverbatim
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*>
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*> \param[in] E
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*> \verbatim
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*> E is REAL array, dimension (N-1)
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*> The (n-1) subdiagonal elements of the tridiagonal matrix A.
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*> \endverbatim
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*>
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*> \param[in] DF
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*> \verbatim
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*> DF is REAL array, dimension (N)
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*> The n diagonal elements of the factor L from the L*D*L'
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*> factorization of A.
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*> \endverbatim
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*>
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*> \param[in] EF
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*> \verbatim
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*> EF is REAL array, dimension (N-1)
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*> The (n-1) subdiagonal elements of the factor L from the
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*> L*D*L' factorization of A.
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*> \endverbatim
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*>
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*> \param[out] WORK
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*> \verbatim
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*> WORK is REAL array, dimension (2*N)
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*> \endverbatim
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*>
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*> \param[out] RESID
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*> \verbatim
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*> RESID is REAL
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*> norm(L*D*L' - A) / (n * norm(A) * EPS)
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*> \endverbatim
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*
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* Authors:
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* ========
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*
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*> \author Univ. of Tennessee
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*> \author Univ. of California Berkeley
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*> \author Univ. of Colorado Denver
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*> \author NAG Ltd.
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*
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*> \ingroup single_lin
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*
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* =====================================================================
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SUBROUTINE SPTT01( N, D, E, DF, EF, WORK, RESID )
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*
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* -- LAPACK test routine --
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* -- LAPACK is a software package provided by Univ. of Tennessee, --
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* -- Univ. of California Berkeley, Univ. of Colorado Denver and NAG Ltd..--
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*
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* .. Scalar Arguments ..
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INTEGER N
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REAL RESID
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* ..
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* .. Array Arguments ..
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REAL D( * ), DF( * ), E( * ), EF( * ), WORK( * )
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* ..
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*
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* =====================================================================
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*
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* .. Parameters ..
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REAL ONE, ZERO
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PARAMETER ( ONE = 1.0E+0, ZERO = 0.0E+0 )
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* ..
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* .. Local Scalars ..
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INTEGER I
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REAL ANORM, DE, EPS
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* ..
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* .. External Functions ..
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REAL SLAMCH
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EXTERNAL SLAMCH
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* ..
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* .. Intrinsic Functions ..
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INTRINSIC ABS, MAX, REAL
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* ..
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* .. Executable Statements ..
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*
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* Quick return if possible
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*
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IF( N.LE.0 ) THEN
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RESID = ZERO
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RETURN
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END IF
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*
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EPS = SLAMCH( 'Epsilon' )
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*
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* Construct the difference L*D*L' - A.
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*
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WORK( 1 ) = DF( 1 ) - D( 1 )
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DO 10 I = 1, N - 1
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DE = DF( I )*EF( I )
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WORK( N+I ) = DE - E( I )
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WORK( 1+I ) = DE*EF( I ) + DF( I+1 ) - D( I+1 )
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10 CONTINUE
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*
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* Compute the 1-norms of the tridiagonal matrices A and WORK.
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*
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IF( N.EQ.1 ) THEN
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ANORM = D( 1 )
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RESID = ABS( WORK( 1 ) )
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ELSE
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ANORM = MAX( D( 1 )+ABS( E( 1 ) ), D( N )+ABS( E( N-1 ) ) )
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RESID = MAX( ABS( WORK( 1 ) )+ABS( WORK( N+1 ) ),
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$ ABS( WORK( N ) )+ABS( WORK( 2*N-1 ) ) )
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DO 20 I = 2, N - 1
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ANORM = MAX( ANORM, D( I )+ABS( E( I ) )+ABS( E( I-1 ) ) )
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RESID = MAX( RESID, ABS( WORK( I ) )+ABS( WORK( N+I-1 ) )+
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$ ABS( WORK( N+I ) ) )
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20 CONTINUE
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END IF
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*
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* Compute norm(L*D*L' - A) / (n * norm(A) * EPS)
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*
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IF( ANORM.LE.ZERO ) THEN
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IF( RESID.NE.ZERO )
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$ RESID = ONE / EPS
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ELSE
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RESID = ( ( RESID / REAL( N ) ) / ANORM ) / EPS
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END IF
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*
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RETURN
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*
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* End of SPTT01
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*
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END
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