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180 lines
4.7 KiB
FortranFixed
180 lines
4.7 KiB
FortranFixed
*> \brief \b DPTTRS
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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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*> \htmlonly
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*> Download DPTTRS + dependencies
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*> <a href="http://www.netlib.org/cgi-bin/netlibfiles.tgz?format=tgz&filename=/lapack/lapack_routine/dpttrs.f">
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*> [TGZ]</a>
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*> <a href="http://www.netlib.org/cgi-bin/netlibfiles.zip?format=zip&filename=/lapack/lapack_routine/dpttrs.f">
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*> [ZIP]</a>
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*> <a href="http://www.netlib.org/cgi-bin/netlibfiles.txt?format=txt&filename=/lapack/lapack_routine/dpttrs.f">
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*> [TXT]</a>
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*> \endhtmlonly
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*
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* Definition:
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* ===========
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*
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* SUBROUTINE DPTTRS( N, NRHS, D, E, B, LDB, INFO )
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*
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* .. Scalar Arguments ..
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* INTEGER INFO, LDB, N, NRHS
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* ..
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* .. Array Arguments ..
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* DOUBLE PRECISION B( LDB, * ), D( * ), E( * )
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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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*> DPTTRS solves a tridiagonal system of the form
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*> A * X = B
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*> using the L*D*L**T factorization of A computed by DPTTRF. D is a
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*> diagonal matrix specified in the vector D, L is a unit bidiagonal
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*> matrix whose subdiagonal is specified in the vector E, and X and B
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*> are N by NRHS matrices.
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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 tridiagonal matrix A. N >= 0.
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*> \endverbatim
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*>
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*> \param[in] NRHS
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*> \verbatim
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*> NRHS is INTEGER
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*> The number of right hand sides, i.e., the number of columns
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*> of the matrix B. NRHS >= 0.
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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 DOUBLE PRECISION array, dimension (N)
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*> The n diagonal elements of the diagonal matrix D from the
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*> L*D*L**T factorization of 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 DOUBLE PRECISION array, dimension (N-1)
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*> The (n-1) subdiagonal elements of the unit bidiagonal factor
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*> L from the L*D*L**T factorization of A. E can also be regarded
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*> as the superdiagonal of the unit bidiagonal factor U from the
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*> factorization A = U**T*D*U.
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*> \endverbatim
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*>
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*> \param[in,out] B
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*> \verbatim
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*> B is DOUBLE PRECISION array, dimension (LDB,NRHS)
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*> On entry, the right hand side vectors B for the system of
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*> linear equations.
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*> On exit, the solution vectors, X.
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*> \endverbatim
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*>
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*> \param[in] LDB
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*> \verbatim
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*> LDB is INTEGER
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*> The leading dimension of the array B. LDB >= max(1,N).
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*> \endverbatim
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*>
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*> \param[out] INFO
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*> \verbatim
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*> INFO is INTEGER
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*> = 0: successful exit
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*> < 0: if INFO = -k, the k-th argument had an illegal value
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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 doublePTcomputational
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*
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* =====================================================================
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SUBROUTINE DPTTRS( N, NRHS, D, E, B, LDB, INFO )
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*
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* -- LAPACK computational 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 INFO, LDB, N, NRHS
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* ..
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* .. Array Arguments ..
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DOUBLE PRECISION B( LDB, * ), D( * ), E( * )
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* ..
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*
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* =====================================================================
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*
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* .. Local Scalars ..
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INTEGER J, JB, NB
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* ..
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* .. External Functions ..
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INTEGER ILAENV
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EXTERNAL ILAENV
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* ..
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* .. External Subroutines ..
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EXTERNAL DPTTS2, XERBLA
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* ..
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* .. Intrinsic Functions ..
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INTRINSIC MAX, MIN
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* ..
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* .. Executable Statements ..
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*
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* Test the input arguments.
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*
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INFO = 0
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IF( N.LT.0 ) THEN
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INFO = -1
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ELSE IF( NRHS.LT.0 ) THEN
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INFO = -2
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ELSE IF( LDB.LT.MAX( 1, N ) ) THEN
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INFO = -6
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END IF
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IF( INFO.NE.0 ) THEN
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CALL XERBLA( 'DPTTRS', -INFO )
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RETURN
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END IF
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*
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* Quick return if possible
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*
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IF( N.EQ.0 .OR. NRHS.EQ.0 )
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$ RETURN
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*
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* Determine the number of right-hand sides to solve at a time.
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*
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IF( NRHS.EQ.1 ) THEN
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NB = 1
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ELSE
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NB = MAX( 1, ILAENV( 1, 'DPTTRS', ' ', N, NRHS, -1, -1 ) )
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END IF
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*
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IF( NB.GE.NRHS ) THEN
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CALL DPTTS2( N, NRHS, D, E, B, LDB )
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ELSE
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DO 10 J = 1, NRHS, NB
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JB = MIN( NRHS-J+1, NB )
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CALL DPTTS2( N, JB, D, E, B( 1, J ), LDB )
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10 CONTINUE
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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 DPTTRS
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*
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END
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