115 lines
3.1 KiB
FortranFixed
115 lines
3.1 KiB
FortranFixed
SUBROUTINE DPTTRS( N, NRHS, D, E, B, LDB, INFO )
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*
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* -- LAPACK routine (version 3.1) --
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* Univ. of Tennessee, Univ. of California Berkeley and NAG Ltd..
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* November 2006
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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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* Purpose
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* =======
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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' 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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*
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* Arguments
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* =========
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*
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* N (input) INTEGER
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* The order of the tridiagonal matrix A. N >= 0.
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*
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* NRHS (input) 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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*
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* D (input) 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' factorization of A.
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*
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* E (input) 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' 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'*D*U.
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*
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* B (input/output) 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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*
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* LDB (input) INTEGER
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* The leading dimension of the array B. LDB >= max(1,N).
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*
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* INFO (output) 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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*
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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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