spttrs.f

Go to the documentation of this file.

      SUBROUTINE SPTTRS( N, NRHS, D, E, B, LDB, INFO )
*
*  -- LAPACK routine (version 3.2) --
*  -- LAPACK is a software package provided by Univ. of Tennessee,    --
*  -- Univ. of California Berkeley, Univ. of Colorado Denver and NAG Ltd..--
*     November 2006
*
*     .. Scalar Arguments ..
      INTEGER            INFO, LDB, N, NRHS
*     ..
*     .. Array Arguments ..
      REAL               B( LDB, * ), D( * ), E( * )
*     ..
*
*  Purpose
*  =======
*
*  SPTTRS solves a tridiagonal system of the form
*     A * X = B
*  using the L*D*L' factorization of A computed by SPTTRF.  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.
*
*  Arguments
*  =========
*
*  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
*          L*D*L' factorization of A.
*
*  E       (input) REAL array, dimension (N-1)
*          The (n-1) subdiagonal elements of the unit bidiagonal factor
*          L from the L*D*L' factorization of A.  E can also be regarded
*          as the superdiagonal of the unit bidiagonal factor U from the
*          factorization A = U'*D*U.
*
*  B       (input/output) REAL array, dimension (LDB,NRHS)
*          On entry, the right hand side vectors B for the system of
*          linear equations.
*          On exit, the solution vectors, X.
*
*  LDB     (input) INTEGER
*          The leading dimension of the array B.  LDB >= max(1,N).
*
*  INFO    (output) INTEGER
*          = 0: successful exit
*          < 0: if INFO = -k, the k-th argument had an illegal value
*
*  =====================================================================
*
*     .. Local Scalars ..
      INTEGER            J, JB, NB
*     ..
*     .. External Functions ..
      INTEGER            ILAENV
      EXTERNAL           ILAENV
*     ..
*     .. External Subroutines ..
      EXTERNAL           SPTTS2, XERBLA
*     ..
*     .. Intrinsic Functions ..
      INTRINSIC          MAX, MIN
*     ..
*     .. Executable Statements ..
*
*     Test the input arguments.
*
      INFO = 0
      IF( N.LT.0 ) THEN
         INFO = -1
      ELSE IF( NRHS.LT.0 ) THEN
         INFO = -2
      ELSE IF( LDB.LT.MAX( 1, N ) ) THEN
         INFO = -6
      END IF
      IF( INFO.NE.0 ) THEN
         CALL XERBLA( 'SPTTRS', -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, 'SPTTRS', ' ', N, NRHS, -1, -1 ) )
      END IF
*
      IF( NB.GE.NRHS ) THEN
         CALL SPTTS2( N, NRHS, D, E, B, LDB )
      ELSE
         DO 10 J = 1, NRHS, NB
            JB = MIN( NRHS-J+1, NB )
            CALL SPTTS2( N, JB, D, E, B( 1, J ), LDB )
   10    CONTINUE
      END IF
*
      RETURN
*
*     End of SPTTRS
*
      END