CXML
        	    
LAPACK version 3.0
dgtrfs(3)

PURPOSE
  DGTRFS - improve the computed solution to a system of linear equations when
  the coefficient matrix is tridiagonal, and provides error bounds and
  backward error estimates for the solution

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

      CHARACTER	     TRANS

      INTEGER	     INFO, LDB, LDX, N, NRHS

      INTEGER	     IPIV( * ), IWORK( * )

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

DESCRIPTION
  DGTRFS improves the computed solution to a system of linear equations when
  the coefficient matrix is tridiagonal, and provides error bounds and
  backward error estimates for the solution.

ARGUMENTS
  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 diagonal elements of A.

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

  DLF	  (input) DOUBLE PRECISION array, dimension (N-1)
	  The (n-1) multipliers that define the matrix L from the LU
	  factorization of A as computed by DGTTRF.

  DF	  (input) DOUBLE PRECISION array, dimension (N)
	  The n diagonal elements of the upper triangular matrix U from the
	  LU factorization of A.

  DUF	  (input) DOUBLE PRECISION array, dimension (N-1)
	  The (n-1) elements of the first superdiagonal of U.

  DU2	  (input) DOUBLE PRECISION array, dimension (N-2)
	  The (n-2) elements of the second superdiagonal 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) DOUBLE PRECISION array, dimension (LDB,NRHS)
	  The right hand side matrix B.

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

  X	  (input/output) DOUBLE PRECISION array, dimension (LDX,NRHS)
	  On entry, the solution matrix X, as computed by DGTTRS.  On exit,
	  the improved solution matrix X.

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

  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

PARAMETERS
  ITMAX is the maximum number of steps of iterative refinement.