DGTRFS(l)		LAPACK routine (version	1.1)		    DGTRFS(l)

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

SYNOPSIS

  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,	* )

PURPOSE
  DGTRFS improves the computed solution	to a system of linear equations	when
  the coefficient matrix is tridiagonal, and provides error bounds and back-
  ward 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 factori-
	  zation 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 inter-
	  changed 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 bounds for each solution vector X(j)
	  (the j-th column of the solution matrix X).  If XTRUE	is the true
	  solution, FERR(j) bounds the magnitude of the	largest	entry in
	  (X(j)	- XTRUE) divided by the	magnitude of the largest entry in
	  X(j).	 The quality of	the error bound	depends	on the quality of the
	  estimate of norm(inv(A)) computed in the code; if the	estimate of
	  norm(inv(A)) is accurate, the	error bound is guaranteed.

  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 entry	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.