sgttrs.f

          SUBROUTINE SGTTRS( TRANS, N, NRHS, DL, D, DU, DU2, IPIV, B, LDB,
         $                   INFO )
    *
    *  -- LAPACK routine (version 3.1) --
    *     Univ. of Tennessee, Univ. of California Berkeley and NAG Ltd..
    *     November 2006
    *
    *     .. Scalar Arguments ..
          CHARACTER          TRANS
          INTEGER            INFO, LDB, N, NRHS
    *     ..
    *     .. Array Arguments ..
          INTEGER            IPIV( * )
          REAL               B( LDB, * ), D( * ), DL( * ), DU( * ), DU2( * )
    *     ..
    *
    *  Purpose
    *  =======
    *
    *  SGTTRS solves one of the systems of equations
    *     A*X = B  or  A'*X = B,
    *  with a tridiagonal matrix A using the LU factorization computed
    *  by SGTTRF.
    *
    *  Arguments
    *  =========
    *
    *  TRANS   (input) CHARACTER*1
    *          Specifies the form of the system of equations.
    *          = 'N':  A * X = B  (No transpose)
    *          = 'T':  A'* X = B  (Transpose)
    *          = 'C':  A'* X = B  (Conjugate transpose = Transpose)
    *
    *  N       (input) INTEGER
    *          The order of the matrix A.
    *
    *  NRHS    (input) INTEGER
    *          The number of right hand sides, i.e., the number of columns
    *          of the matrix B.  NRHS >= 0.
    *
    *  DL      (input) REAL array, dimension (N-1)
    *          The (n-1) multipliers that define the matrix L from the
    *          LU factorization of A.
    *
    *  D       (input) REAL array, dimension (N)
    *          The n diagonal elements of the upper triangular matrix U from
    *          the LU factorization of A.
    *
    *  DU      (input) REAL array, dimension (N-1)
    *          The (n-1) elements of the first super-diagonal of U.
    *
    *  DU2     (input) REAL array, dimension (N-2)
    *          The (n-2) elements of the second super-diagonal of U.
    *
    *  IPIV    (input) INTEGER array, dimension (N)
    *          The pivot indices; for 1 <= i <= n, row i of the matrix was
    *          interchanged with row IPIV(i).  IPIV(i) will always be either
    *          i or i+1; IPIV(i) = i indicates a row interchange was not
    *          required.
    *
    *  B       (input/output) REAL array, dimension (LDB,NRHS)
    *          On entry, the matrix of right hand side vectors B.
    *          On exit, B is overwritten by 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 = -i, the i-th argument had an illegal value
    *
    *  =====================================================================
    *
    *     .. Local Scalars ..
          LOGICAL            NOTRAN
          INTEGER            ITRANS, J, JB, NB
    *     ..
    *     .. External Functions ..
          INTEGER            ILAENV
          EXTERNAL           ILAENV
    *     ..
    *     .. External Subroutines ..
          EXTERNAL           SGTTS2, XERBLA
    *     ..
    *     .. Intrinsic Functions ..
          INTRINSIC          MAX, MIN
    *     ..
    *     .. Executable Statements ..
    *
          INFO = 0
          NOTRAN = ( TRANS.EQ.'N' .OR. TRANS.EQ.'n' )
          IF( .NOT.NOTRAN .AND. .NOT.( TRANS.EQ.'T' .OR. TRANS.EQ.
         $    't' ) .AND. .NOT.( TRANS.EQ.'C' .OR. TRANS.EQ.'c' ) ) THEN
             INFO = -1
          ELSE IF( N.LT.0 ) THEN
             INFO = -2
          ELSE IF( NRHS.LT.0 ) THEN
             INFO = -3
          ELSE IF( LDB.LT.MAX( N, 1 ) ) THEN
             INFO = -10
          END IF
          IF( INFO.NE.0 ) THEN
             CALL XERBLA( 'SGTTRS', -INFO )
             RETURN
          END IF
    *
    *     Quick return if possible
    *
          IF( N.EQ.0 .OR. NRHS.EQ.0 )
         $   RETURN
    *
    *     Decode TRANS
    *
          IF( NOTRAN ) THEN
             ITRANS = 0
          ELSE
             ITRANS = 1
          END IF
    *
    *     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, 'SGTTRS', TRANS, N, NRHS, -1, -1 ) )
          END IF
    *
          IF( NB.GE.NRHS ) THEN
             CALL SGTTS2( ITRANS, N, NRHS, DL, D, DU, DU2, IPIV, B, LDB )
          ELSE
             DO 10 J = 1, NRHS, NB
                JB = MIN( NRHS-J+1, NB )
                CALL SGTTS2( ITRANS, N, JB, DL, D, DU, DU2, IPIV, B( 1, J ),
         $                   LDB )
       10    CONTINUE
          END IF
    *
    *     End of SGTTRS
    *
          END