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SUBROUTINE DPTTRSV( TRANS, N, NRHS, D, E, B, LDB,
$ INFO )
*
* Written by Andrew J. Cleary, University of Tennessee.
* November, 1996.
* Modified from DPTTRS:
* -- LAPACK routine (preliminary version) --
* Univ. of Tennessee, Univ. of California Berkeley, NAG Ltd.,
* Courant Institute, Argonne National Lab, and Rice University
*
* .. Scalar Arguments ..
CHARACTER TRANS
INTEGER INFO, LDB, N, NRHS
* ..
* .. Array Arguments ..
DOUBLE PRECISION D( * )
DOUBLE PRECISION B( LDB, * ), E( * )
* ..
*
* Purpose
* =======
*
* DPTTRSV solves one of the triangular systems
* L**T* X = B, or L * X = B,
* where L is the Cholesky factor of a Hermitian positive
* definite tridiagonal matrix A such that
* A = L*D*L**H (computed by DPTTRF).
*
* Arguments
* =========
*
* TRANS (input) CHARACTER
* Specifies the form of the system of equations:
* = 'N': L * X = B (No transpose)
* = 'T': L**T * X = B (Transpose)
*
* N (input) INTEGER
* The order of the tridiagonal matrix A. N >= 0.
*
* NRHS (input) INTEGER
* The number of right hand sides, i.e., the number of columns
* of the matrix B. NRHS >= 0.
*
* D (input) REAL array, dimension (N)
* The n diagonal elements of the diagonal matrix D from the
* factorization computed by DPTTRF.
*
* E (input) COMPLEX array, dimension (N-1)
* The (n-1) off-diagonal elements of the unit bidiagonal
* factor U or L from the factorization computed by DPTTRF
* (see UPLO).
*
* B (input/output) COMPLEX array, dimension (LDB,NRHS)
* On entry, the right hand side matrix B.
* On exit, the solution matrix X.
*
* LDB (input) INTEGER
* The leading dimension of the array B. LDB >= max(1,N).
*
* INFO (output) INTEGER
* = 0: successful exit
* < 0: if INFO = -i, the i-th argument had an illegal value
*
* =====================================================================
*
* .. Local Scalars ..
LOGICAL NOTRAN
INTEGER I, J
* ..
* .. External Functions ..
LOGICAL LSAME
EXTERNAL LSAME
* ..
* .. External Subroutines ..
EXTERNAL XERBLA
* ..
* .. Intrinsic Functions ..
INTRINSIC MAX
* ..
* .. Executable Statements ..
*
* Test the input arguments.
*
INFO = 0
NOTRAN = LSAME( TRANS, 'N' )
IF( .NOT.NOTRAN .AND. .NOT.LSAME( TRANS, 'T' ) ) 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( 'DPTTRS', -INFO )
RETURN
END IF
*
* Quick return if possible
*
IF( N.EQ.0 )
$ RETURN
IF( NOTRAN ) THEN
*
DO 60 J = 1, NRHS
*
* Solve L * x = b.
*
DO 40 I = 2, N
B( I, J ) = B( I, J ) - B( I-1, J )*E( I-1 )
40 CONTINUE
60 CONTINUE
*
ELSE
*
DO 65 J = 1, NRHS
*
* Solve L**H * x = b.
*
DO 50 I = N - 1, 1, -1
B( I, J ) = B( I, J ) -
$ B( I+1, J )*( E( I ) )
50 CONTINUE
65 CONTINUE
ENDIF
*
RETURN
*
* End of DPTTRS
*
END
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