dptts2()

subroutine dptts2	(	integer	n,
		integer	nrhs,
		double precision, dimension( * )	d,
		double precision, dimension( * )	e,
		double precision, dimension( ldb, * )	b,
		integer	ldb )

DPTTS2 solves a tridiagonal system of the form AX=B using the L D LH factorization computed by spttrf.

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Purpose:

!>
!> DPTTS2 solves a tridiagonal system of the form
!>    A * X = B
!> using the L*D*L**T factorization of A computed by DPTTRF.  D is a
!> diagonal matrix specified in the vector D, L is a unit bidiagonal
!> matrix whose subdiagonal is specified in the vector E, and X and B
!> are N by NRHS matrices.
!> 

Parameters

[in]	N	!> N is INTEGER !> The order of the tridiagonal matrix A. N >= 0. !>
[in]	NRHS	!> NRHS is INTEGER !> The number of right hand sides, i.e., the number of columns !> of the matrix B. NRHS >= 0. !>
[in]	D	!> D is DOUBLE PRECISION array, dimension (N) !> The n diagonal elements of the diagonal matrix D from the !> L*D*L**T factorization of A. !>
[in]	E	!> E is DOUBLE PRECISION array, dimension (N-1) !> The (n-1) subdiagonal elements of the unit bidiagonal factor !> L from the L*D*L**T factorization of A. E can also be regarded !> as the superdiagonal of the unit bidiagonal factor U from the !> factorization A = U**T*D*U. !>
[in,out]	B	!> B is DOUBLE PRECISION array, dimension (LDB,NRHS) !> On entry, the right hand side vectors B for the system of !> linear equations. !> On exit, the solution vectors, X. !>
[in]	LDB	!> LDB is INTEGER !> The leading dimension of the array B. LDB >= max(1,N). !>

Author

Univ. of Tennessee

Univ. of California Berkeley

Univ. of Colorado Denver

NAG Ltd.

Definition at line 99 of file dptts2.f.

*
*  -- LAPACK computational routine --
*  -- LAPACK is a software package provided by Univ. of Tennessee,    --
*  -- Univ. of California Berkeley, Univ. of Colorado Denver and NAG Ltd..--
*
*     .. Scalar Arguments ..
      INTEGER            LDB, N, NRHS
*     ..
*     .. Array Arguments ..
      DOUBLE PRECISION   B( LDB, * ), D( * ), E( * )
*     ..
*
*  =====================================================================
*
*     .. Local Scalars ..
      INTEGER            I, J
*     ..
*     .. External Subroutines ..
      EXTERNAL           dscal
*     ..
*     .. Executable Statements ..
*
*     Quick return if possible
*
      IF( n.LE.1 ) THEN
         IF( n.EQ.1 )
     $      CALL dscal( nrhs, 1.d0 / d( 1 ), b, ldb )
         RETURN
      END IF
*
*     Solve A * X = B using the factorization A = L*D*L**T,
*     overwriting each right hand side vector with its solution.
*
      DO 30 j = 1, nrhs
*
*           Solve L * x = b.
*
         DO 10 i = 2, n
            b( i, j ) = b( i, j ) - b( i-1, j )*e( i-1 )
   10    CONTINUE
*
*           Solve D * L**T * x = b.
*
         b( n, j ) = b( n, j ) / d( n )
         DO 20 i = n - 1, 1, -1
            b( i, j ) = b( i, j ) / d( i ) - b( i+1, j )*e( i )
   20    CONTINUE
   30 CONTINUE
*
      RETURN
*
*     End of DPTTS2
*

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