Provided by: liblapack-doc_3.3.1-1_all bug

NAME

       LAPACK-3  -  uses  the LU factorization to compute the solution to a real system of linear
       equations A * X = B or A**T * X = B,

SYNOPSIS

       SUBROUTINE DGTSVX( FACT, TRANS, N, NRHS, DL, D, DU, DLF, DF, DUF, DU2, IPIV,  B,  LDB,  X,
                          LDX, RCOND, FERR, BERR, WORK, IWORK, INFO )

           CHARACTER      FACT, TRANS

           INTEGER        INFO, LDB, LDX, N, NRHS

           DOUBLE         PRECISION RCOND

           INTEGER        IPIV( * ), IWORK( * )

           DOUBLE         PRECISION  B(  LDB, * ), BERR( * ), D( * ), DF( * ), DL( * ), DLF( * ),
                          DU( * ), DU2( * ), DUF( * ), FERR( * ), WORK( * ), X( LDX, * )

PURPOSE

       DGTSVX uses the LU factorization to compute the  solution  to  a  real  system  of  linear
       equations A * X = B or A**T * X = B,
        where A is a tridiagonal matrix of order N and X and B are N-by-NRHS
        matrices.
        Error bounds on the solution and a condition estimate are also
        provided.

DESCRIPTION

        The following steps are performed:
        1. If FACT = 'N', the LU decomposition is used to factor the matrix A
           as A = L * U, where L is a product of permutation and unit lower
           bidiagonal matrices and U is upper triangular with nonzeros in
           only the main diagonal and first two superdiagonals.
        2. If some U(i,i)=0, so that U is exactly singular, then the routine
           returns with INFO = i. Otherwise, the factored form of A is used
           to estimate the condition number of the matrix A.  If the
           reciprocal of the condition number is less than machine precision,
           INFO = N+1 is returned as a warning, but the routine still goes on
           to solve for X and compute error bounds as described below.
        3. The system of equations is solved for X using the factored form
           of A.
        4. Iterative refinement is applied to improve the computed solution
           matrix and calculate error bounds and backward error estimates
           for it.

ARGUMENTS

        FACT    (input) CHARACTER*1
                Specifies whether or not the factored form of A has been
                supplied on entry.
                = 'F':  DLF, DF, DUF, DU2, and IPIV contain the factored
                form of A; DL, D, DU, DLF, DF, DUF, DU2 and IPIV
                will not be modified.
                = 'N':  The matrix will be copied to DLF, DF, and DUF
                and factored.

        TRANS   (input) CHARACTER*1
                Specifies the form of the system of equations:
                = 'N':  A * X = B     (No transpose)
                = 'T':  A**T * X = B  (Transpose)
                = 'C':  A**H * X = B  (Conjugate transpose = Transpose)

        N       (input) INTEGER
                The order of the 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.

        DL      (input) DOUBLE PRECISION array, dimension (N-1)
                The (n-1) subdiagonal elements of A.

        D       (input) DOUBLE PRECISION array, dimension (N)
                The n diagonal elements of A.

        DU      (input) DOUBLE PRECISION array, dimension (N-1)
                The (n-1) superdiagonal elements of A.

        DLF     (input or output) DOUBLE PRECISION array, dimension (N-1)
                If FACT = 'F', then DLF is an input argument and on entry
                contains the (n-1) multipliers that define the matrix L from
                the LU factorization of A as computed by DGTTRF.
                If FACT = 'N', then DLF is an output argument and on exit
                contains the (n-1) multipliers that define the matrix L from
                the LU factorization of A.

        DF      (input or output) DOUBLE PRECISION array, dimension (N)
                If FACT = 'F', then DF is an input argument and on entry
                contains the n diagonal elements of the upper triangular
                matrix U from the LU factorization of A.
                If FACT = 'N', then DF is an output argument and on exit
                contains the n diagonal elements of the upper triangular
                matrix U from the LU factorization of A.

        DUF     (input or output) DOUBLE PRECISION array, dimension (N-1)
                If FACT = 'F', then DUF is an input argument and on entry
                contains the (n-1) elements of the first superdiagonal of U.
                If FACT = 'N', then DUF is an output argument and on exit
                contains the (n-1) elements of the first superdiagonal of U.

        DU2     (input or output) DOUBLE PRECISION array, dimension (N-2)
                If FACT = 'F', then DU2 is an input argument and on entry
                contains the (n-2) elements of the second superdiagonal of
                U.
                If FACT = 'N', then DU2 is an output argument and on exit
                contains the (n-2) elements of the second superdiagonal of
                U.

        IPIV    (input or output) INTEGER array, dimension (N)
                If FACT = 'F', then IPIV is an input argument and on entry
                contains the pivot indices from the LU factorization of A as
                computed by DGTTRF.
                If FACT = 'N', then IPIV is an output argument and on exit
                contains the pivot indices from the LU factorization of A;
                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) DOUBLE PRECISION array, dimension (LDB,NRHS)
                The N-by-NRHS right hand side matrix B.

        LDB     (input) INTEGER
                The leading dimension of the array B.  LDB >= max(1,N).

        X       (output) DOUBLE PRECISION array, dimension (LDX,NRHS)
                If INFO = 0 or INFO = N+1, the N-by-NRHS solution matrix X.

        LDX     (input) INTEGER
                The leading dimension of the array X.  LDX >= max(1,N).

        RCOND   (output) DOUBLE PRECISION
                The estimate of the reciprocal condition number of the matrix
                A.  If RCOND is less than the machine precision (in
                particular, if RCOND = 0), the matrix is singular to working
                precision.  This condition is indicated by a return code of
                INFO > 0.

        FERR    (output) DOUBLE PRECISION array, dimension (NRHS)
                The estimated forward error bound for each solution vector
                X(j) (the j-th column of the solution matrix X).
                If XTRUE is the true solution corresponding to X(j), FERR(j)
                is an estimated upper bound for the magnitude of the largest
                element in (X(j) - XTRUE) divided by the magnitude of the
                largest element in X(j).  The estimate is as reliable as
                the estimate for RCOND, and is almost always a slight
                overestimate of the true error.

        BERR    (output) DOUBLE PRECISION array, dimension (NRHS)
                The componentwise relative backward error of each solution
                vector X(j) (i.e., the smallest relative change in
                any element of A or B that makes X(j) an exact solution).

        WORK    (workspace) DOUBLE PRECISION array, dimension (3*N)

        IWORK   (workspace) INTEGER array, dimension (N)

        INFO    (output) INTEGER
                = 0:  successful exit
                < 0:  if INFO = -i, the i-th argument had an illegal value
                > 0:  if INFO = i, and i is
                <= N:  U(i,i) is exactly zero.  The factorization
                has not been completed unless i = N, but the
                factor U is exactly singular, so the solution
                and error bounds could not be computed.
                RCOND = 0 is returned.
                = N+1: U is nonsingular, but RCOND is less than machine
                precision, meaning that the matrix is singular
                to working precision.  Nevertheless, the
                solution and error bounds are computed because
                there are a number of situations where the
                computed solution can be more accurate than the
                value of RCOND would suggest.

 LAPACK routine (version 3.2)               April 2011                            DGTSVX(3lapack)