zpttrs()

subroutine zpttrs	(	character	uplo,
		integer	n,
		integer	nrhs,
		double precision, dimension( * )	d,
		complex*16, dimension( * )	e,
		complex*16, dimension( ldb, * )	b,
		integer	ldb,
		integer	info )

ZPTTRS

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Purpose:

!>
!> ZPTTRS solves a tridiagonal system of the form
!>    A * X = B
!> using the factorization A = U**H *D* U or A = L*D*L**H computed by ZPTTRF.
!> D is a diagonal matrix specified in the vector D, U (or L) is a unit
!> bidiagonal matrix whose superdiagonal (subdiagonal) is specified in
!> the vector E, and X and B are N by NRHS matrices.
!> 

Parameters

[in]	UPLO	!> UPLO is CHARACTER*1 !> Specifies the form of the factorization and whether the !> vector E is the superdiagonal of the upper bidiagonal factor !> U or the subdiagonal of the lower bidiagonal factor L. !> = 'U': A = U**H *D*U, E is the superdiagonal of U !> = 'L': A = L*D*L**H, E is the subdiagonal of L !>
[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 !> factorization A = U**H *D*U or A = L*D*L**H. !>
[in]	E	!> E is COMPLEX*16 array, dimension (N-1) !> If UPLO = 'U', the (n-1) superdiagonal elements of the unit !> bidiagonal factor U from the factorization A = U**H*D*U. !> If UPLO = 'L', the (n-1) subdiagonal elements of the unit !> bidiagonal factor L from the factorization A = L*D*L**H. !>
[in,out]	B	!> B is COMPLEX*16 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). !>
[out]	INFO	!> INFO is INTEGER !> = 0: successful exit !> < 0: if INFO = -k, the k-th argument had an illegal value !>

Author

Univ. of Tennessee

Univ. of California Berkeley

Univ. of Colorado Denver

NAG Ltd.

Definition at line 118 of file zpttrs.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 ..
      CHARACTER          UPLO
      INTEGER            INFO, LDB, N, NRHS
*     ..
*     .. Array Arguments ..
      DOUBLE PRECISION   D( * )
      COMPLEX*16         B( LDB, * ), E( * )
*     ..
*
*  =====================================================================
*
*     .. Local Scalars ..
      LOGICAL            UPPER
      INTEGER            IUPLO, J, JB, NB
*     ..
*     .. External Functions ..
      INTEGER            ILAENV
      EXTERNAL           ilaenv
*     ..
*     .. External Subroutines ..
      EXTERNAL           xerbla, zptts2
*     ..
*     .. Intrinsic Functions ..
      INTRINSIC          max, min
*     ..
*     .. Executable Statements ..
*
*     Test the input arguments.
*
      info = 0
      upper = ( uplo.EQ.'U' .OR. uplo.EQ.'u' )
      IF( .NOT.upper .AND. .NOT.( uplo.EQ.'L' .OR. uplo.EQ.'l' ) ) THEN
         info = -1
      ELSE IF( n.LT.0 ) THEN
         info = -2
      ELSE IF( nrhs.LT.0 ) THEN
         info = -3
      ELSE IF( ldb.LT.max( 1, n ) ) THEN
         info = -7
      END IF
      IF( info.NE.0 ) THEN
         CALL xerbla( 'ZPTTRS', -info )
         RETURN
      END IF
*
*     Quick return if possible
*
      IF( n.EQ.0 .OR. nrhs.EQ.0 )
     $   RETURN
*
*     Determine the number of right-hand sides to solve at a time.
*
      IF( nrhs.EQ.1 ) THEN
         nb = 1
      ELSE
         nb = max( 1, ilaenv( 1, 'ZPTTRS', uplo, n, nrhs, -1, -1 ) )
      END IF
*
*     Decode UPLO
*
      IF( upper ) THEN
         iuplo = 1
      ELSE
         iuplo = 0
      END IF
*
      IF( nb.GE.nrhs ) THEN
         CALL zptts2( iuplo, n, nrhs, d, e, b, ldb )
      ELSE
         DO 10 j = 1, nrhs, nb
            jb = min( nrhs-j+1, nb )
            CALL zptts2( iuplo, n, jb, d, e, b( 1, j ), ldb )
   10    CONTINUE
      END IF
*
      RETURN
*
*     End of ZPTTRS
*

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