spttrs.f

          SUBROUTINE SPTTRS( N, NRHS, D, E, B, LDB, INFO )
    *
    *  -- LAPACK routine (version 3.1) --
    *     Univ. of Tennessee, Univ. of California Berkeley 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